From bb5cc8f7951cb162446968f21a8ade2074148964 Mon Sep 17 00:00:00 2001
From: Maurice <mauriceboe@icloud.com>
Date: Thu, 8 Oct 2026 11:59:16 +0200
Subject: [PATCH] fix(db): one account per email address, whatever its case

Login, password reset and the sign-up checks all match emails
case-insensitively, but the only constraint was the column's own
UNIQUE, which SQLite compares case-sensitively. Two accounts whose
addresses differ only in case (legacy rows, two sign-ups at once) left a
login to resolve to whichever row came first, and LOWER(email) lookups
had no index to use.

A migration adds a unique index on lower(email). An install that
already holds such a pair must still start, so the migration then
leaves the index out, logs how many addresses are shared and by which
user ids (not the addresses), and records email_case_index_pending.
Since a migration runs once, db/email-case-index.ts runs on every boot
while that flag is set: it repeats the warning until the accounts are
told apart, then creates the index and clears the flag. The app-level
checks keep guarding meanwhile.

A write that passes the up-front check and still loses the race is now
refused by the database. The users repository turns that unique
violation into UserIdentityTakenError, and sign-up, admin create and
edit, and the profile update answer it with the same 409 their checks
give, instead of a 500 or an unhandled rejection (ADMIN-SVC-094 pinned
the old behaviour and now pins the 409).

The entity generator could not see an expression index (PRAGMA
index_info names its column NULL), so PARITY-009c found a db index with
no entity declaration. Rule 14 carries such an index into the entity as
a named entry with its own statement; Users.entity.ts is regenerated.
---
 server/CLAUDE.md                              |  2 +-
 server/scripts/generate-entities.ts           | 74 +++++++++++++-
 server/src/db/email-case-index.ts             | 38 +++++++
 server/src/db/entities/Users.entity.ts        |  4 +
 ...one_account_per_email_whatever_its_case.ts | 45 +++++++++
 server/src/db/orm.ts                          |  3 +
 .../src/db/repositories/Users.repository.ts   | 69 +++++++++----
 server/src/nest/admin/admin.service.ts        | 55 +++++++----
 server/src/nest/auth/auth.service.ts          |  9 +-
 server/src/nest/auth/user-profile.service.ts  | 18 +++-
 server/tests/unit/db/email-case-index.test.ts | 99 +++++++++++++++++++
 .../tests/unit/db/generate-entities.test.ts   | 28 ++++++
 server/tests/unit/db/legacy-baseline.test.ts  |  1 +
 .../db/repositories/Users.repository.test.ts  | 23 +++++
 server/tests/unit/nest/admin.service.test.ts  | 41 ++++++--
 server/tests/unit/nest/auth.service.test.ts   | 16 +++
 .../unit/nest/user-profile.service.test.ts    | 15 +++
 wiki/Admin-Users-and-Invites.md               |  4 +
 18 files changed, 488 insertions(+), 56 deletions(-)
 create mode 100644 server/src/db/email-case-index.ts
 create mode 100644 server/src/db/migrations/Migration20200101042400_one_account_per_email_whatever_its_case.ts
 create mode 100644 server/tests/unit/db/email-case-index.test.ts

diff --git a/server/CLAUDE.md b/server/CLAUDE.md
index d985fca7c..3389809ef 100644
--- a/server/CLAUDE.md
+++ b/server/CLAUDE.md
@@ -33,7 +33,7 @@ Nest owns everything. Every domain is a DI module under `src/nest/<domain>/` (`c
 - **Scheduling**: `src/nest/scheduling/` (`CronRegistrarService`) is the one way to schedule a cron. Jobs are `*.job.ts` providers in their owning domain. The registrar refuses to schedule under `NODE_ENV=test` (an integration test pins it). **Never use a `@Cron` decorator** — it bypasses that gate.
 - **Plugins** (`src/nest/plugins/`): the child-process runtime (`supervisor/`, `runtime/plugin-host-entry.ts`, `protocol/envelope.ts`) is **intentionally not Nest** — it is the security boundary. `host/rpc-host.ts` registers an RPC method at spawn only if the plugin holds the unlocking permission (registration = authorization). Install-time gates live in `install/`; the manifest's `trek` version range fails closed, and `TREK_PLUGINS_IGNORE_TREK_RANGE` downgrades that to a warning. The protocol tables in `protocol/envelope.ts` (+ `src/plugin-event-sink.ts`) are **generated** into `plugin-sdk/` and `shared/` — run `gen:plugin-facts` after touching them and commit the output. `runtime/egress-policy.ts` is a hand-kept mirror guarded by a parity test in `plugin-sdk/`; change both sides together.
 - **Storage**: `src/nest/storage/` is the upload driver abstraction (local / S3 / mirror behind one interface, with an admin surface and a usage job). Domain services never touch the filesystem or the S3 SDK directly.
-- **DB** (`src/db/`): `better-sqlite3`, WAL, FK on. `database.ts` only opens the connection and applies pragmas — it does not build a schema. MikroORM's migrations (`db/migrations/`) are the only schema source, **including for tests**: the hand-kept `schema.ts`/`migrations.ts`/`seeds.ts` builders and the parity test that guarded them are gone. Seeding is the MikroORM seeders in `db/seeders/`, applied by `db/orm.ts` from `buildApp()` before anything reads. An install the retired positional runner migrated (a `schema_version` row, no `mikro_orm_migrations` rows) is baselined by `db/legacy-baseline.ts`: the migrations whose `Legacy migration step N` docstring is ≤ its version are recorded as executed (never replayed) in the same transaction as the migrator run, while the baseline migration itself still runs first, as the legacy runner's `createTables()` did on every boot; anything it cannot map refuses the boot. Before a boot applies pending migrations to a file database that holds anything, `db/pre-migrate-snapshot.ts` writes a `VACUUM INTO` copy next to it (`pre-migrate-<schema>-<time>.db`, `<schema>` being `legacy-<schema_version>` or the last recorded migration's timestamp; the newest `TREK_DB_PRE_MIGRATE_SNAPSHOT_KEEP` stay) and the newer-database refusal names the matching copy; a copy that cannot be written refuses the boot unless `TREK_DB_PRE_MIGRATE_SNAPSHOT=false`. It is off under `NODE_ENV=test` unless a suite sets that variable to true, and a restore's re-bootstrap skips it (the archive is the copy). `db/orm-driver.ts` binds the ORM to that same connection instead of opening a second one — with `:memory:` under test, a second connection is a second, empty database. Every read and write goes through a repository in `db/repositories/` (each extends the shared `TrekRepository` base in `db/repositories/_shared/`, which validates the request context before delegating to MikroORM) and, for multi-statement writes, `UnitOfWork.transactional(...)` (`nest/database/unit-of-work.ts`, provided by the global `OrmModule`) — there is no `DatabaseService`/`DatabaseModule` to inject any more. Entrypoints outside an HTTP request — an MCP tool call, a plugin RPC dispatch, a WebSocket message, a cron job, boot-time seeding, a backup restore — open one explicitly with `withRequestContext()` (`nest/database/request-context.ts`), since only the pathless `mikroOrmRequestContext` middleware `buildApp()` mounts (with `@mikro-orm/nestjs`'s own `registerRequestContext` turned off in `app.module.ts`, UPLOADS-P16) forks an `EntityManager` per HTTP request. `NODE_ENV=test` ⇒ isolated `:memory:` DB per vitest worker, opened from a snapshot `tests/global-setup.ts` migrates once per run; `TREK_DB_FILE` ⇒ explicit file DB (Playwright harness).
+- **DB** (`src/db/`): `better-sqlite3`, WAL, FK on. `database.ts` only opens the connection and applies pragmas — it does not build a schema. MikroORM's migrations (`db/migrations/`) are the only schema source, **including for tests**: the hand-kept `schema.ts`/`migrations.ts`/`seeds.ts` builders and the parity test that guarded them are gone. Seeding is the MikroORM seeders in `db/seeders/`, applied by `db/orm.ts` from `buildApp()` before anything reads. An install the retired positional runner migrated (a `schema_version` row, no `mikro_orm_migrations` rows) is baselined by `db/legacy-baseline.ts`: the migrations whose `Legacy migration step N` docstring is ≤ its version are recorded as executed (never replayed) in the same transaction as the migrator run, while the baseline migration itself still runs first, as the legacy runner's `createTables()` did on every boot; anything it cannot map refuses the boot. Before a boot applies pending migrations to a file database that holds anything, `db/pre-migrate-snapshot.ts` writes a `VACUUM INTO` copy next to it (`pre-migrate-<schema>-<time>.db`, `<schema>` being `legacy-<schema_version>` or the last recorded migration's timestamp; the newest `TREK_DB_PRE_MIGRATE_SNAPSHOT_KEEP` stay) and the newer-database refusal names the matching copy; a copy that cannot be written refuses the boot unless `TREK_DB_PRE_MIGRATE_SNAPSHOT=false`. It is off under `NODE_ENV=test` unless a suite sets that variable to true, and a restore's re-bootstrap skips it (the archive is the copy). After the migrator, `db/email-case-index.ts` creates the case-insensitive unique index on `users.email` that its migration had to defer on an install holding two accounts whose emails differ only in case (flag `email_case_index_pending`). `db/orm-driver.ts` binds the ORM to that same connection instead of opening a second one — with `:memory:` under test, a second connection is a second, empty database. Every read and write goes through a repository in `db/repositories/` (each extends the shared `TrekRepository` base in `db/repositories/_shared/`, which validates the request context before delegating to MikroORM) and, for multi-statement writes, `UnitOfWork.transactional(...)` (`nest/database/unit-of-work.ts`, provided by the global `OrmModule`) — there is no `DatabaseService`/`DatabaseModule` to inject any more. Entrypoints outside an HTTP request — an MCP tool call, a plugin RPC dispatch, a WebSocket message, a cron job, boot-time seeding, a backup restore — open one explicitly with `withRequestContext()` (`nest/database/request-context.ts`), since only the pathless `mikroOrmRequestContext` middleware `buildApp()` mounts (with `@mikro-orm/nestjs`'s own `registerRequestContext` turned off in `app.module.ts`, UPLOADS-P16) forks an `EntityManager` per HTTP request. `NODE_ENV=test` ⇒ isolated `:memory:` DB per vitest worker, opened from a snapshot `tests/global-setup.ts` migrates once per run; `TREK_DB_FILE` ⇒ explicit file DB (Playwright harness).
 
 ## Rules for new code
 
diff --git a/server/scripts/generate-entities.ts b/server/scripts/generate-entities.ts
index f7f10c726..3528501e9 100644
--- a/server/scripts/generate-entities.ts
+++ b/server/scripts/generate-entities.ts
@@ -768,6 +768,41 @@ export function RULE9_addImplicitUniqueConstraints(
   }
 }
 
+/** One index over an expression rather than plain columns, as Rule 14 hands it in. */
+export interface ExpressionIndex {
+  name: string;
+  unique: boolean;
+  /** The `CREATE [UNIQUE] INDEX` statement SQLite stored for it. */
+  sql: string;
+}
+
+/**
+ * Rule 14 (metadata level; DB-dependent half in `collectExpressionIndexes`):
+ * an index over an expression (`idx_users_email_lower ON users (lower(email))`)
+ * becomes a named `indexes:`/`uniques:` entry carrying its own `expression`.
+ *
+ * `SqliteSchemaHelper.getIndexes()` reads columns from `PRAGMA index_info`,
+ * which reports an expression column with a NULL name, so the introspected
+ * index has no column the generator can map and it never reaches the entity.
+ * PARITY-009c would then find a db index without an entity declaration. Any
+ * introspected entry under the same name is replaced, so the rule holds even
+ * if a later MikroORM keeps the broken entry instead of dropping it.
+ */
+export function RULE14_addExpressionIndexes(
+  metadata: EntityMetadata[],
+  expressionIndexes: ReadonlyMap<string, ExpressionIndex[]>,
+): void {
+  for (const meta of metadata) {
+    const indexes = expressionIndexes.get(meta.tableName);
+    if (!indexes) continue;
+    for (const index of indexes) {
+      meta.indexes = meta.indexes.filter((existing) => existing.name !== index.name);
+      meta.uniques = meta.uniques.filter((existing) => existing.name !== index.name);
+      (index.unique ? meta.uniques : meta.indexes).push({ name: index.name, expression: index.sql });
+    }
+  }
+}
+
 /** A repository-type marker to inject into one entity class — Rule 10. */
 export interface RepositoryTypeMarkerFixup {
   className: string;
@@ -1210,6 +1245,7 @@ export function applyRules(
   _platform: Platform,
   implicitUniques: ReadonlyMap<string, string[][]> = new Map(),
   fkDeleteRules: ReadonlyMap<string, ReadonlyMap<string, string>> = new Map(),
+  expressionIndexes: ReadonlyMap<string, ExpressionIndex[]> = new Map(),
 ): RuleFixups {
   const retypedByRule1 = RULE1_fixUnknownScalarTypes(metadata);
   const jsonColumns = RULE1b_markJsonColumns(metadata);
@@ -1230,6 +1266,9 @@ export function applyRules(
   RULE7_dropNoActionRules(metadata);
   RULE8_bindRepositories(metadata);
   RULE9_addImplicitUniqueConstraints(metadata, implicitUniques);
+  // Like RULE9 it only appends to `meta.indexes`/`meta.uniques`, which the
+  // renderer dumps verbatim; no text pass depends on it.
+  RULE14_addExpressionIndexes(metadata, expressionIndexes);
   const defaults = RULE_normalizeLiteralDefaults(metadata);
   const repositoryMarkers = RULE10_repositoryTypeMarker(metadata);
   // Runs after RULE5 so it reads the FINAL (post-rename) owning-relation
@@ -1296,6 +1335,38 @@ export async function collectImplicitUniqueIndexes(
   return result;
 }
 
+/**
+ * DB-dependent half of Rule 14: every named index whose `PRAGMA index_info`
+ * has an expression column (a NULL name), with the statement `sqlite_master`
+ * stored for it, so `RULE14_addExpressionIndexes` stays a pure function.
+ */
+export async function collectExpressionIndexes(
+  connection: Connection,
+  tableNames: readonly string[],
+): Promise<Map<string, ExpressionIndex[]>> {
+  const result = new Map<string, ExpressionIndex[]>();
+  for (const tableName of tableNames) {
+    const indexList = (await connection.execute(`pragma index_list(\`${tableName}\`)`, [], 'all')) as {
+      name: string;
+      unique: number;
+    }[];
+    const perTable: ExpressionIndex[] = [];
+    for (const idx of indexList) {
+      if (idx.name.startsWith('sqlite_autoindex_')) continue;
+      const info = (await connection.execute(`pragma index_info(\`${idx.name}\`)`, [], 'all')) as {
+        name: string | null;
+      }[];
+      if (!info.some((column) => column.name === null)) continue;
+      const rows = (await connection.execute(`select sql from sqlite_master where type = 'index' and name = ?`, [idx.name], 'all')) as {
+        sql: string;
+      }[];
+      perTable.push({ name: idx.name, unique: idx.unique === 1, sql: rows[0].sql });
+    }
+    if (perTable.length > 0) result.set(tableName, perTable);
+  }
+  return result;
+}
+
 /**
  * DB-dependent half of Rule 13: for every table name given, `PRAGMA
  * foreign_key_list`'s `on_delete` per `from` (owning) column, lowercased —
@@ -1953,7 +2024,8 @@ export async function generateEntities(): Promise<GenerateResult> {
           const tableNames = metadata.map((meta) => meta.tableName);
           const implicitUniques = await collectImplicitUniqueIndexes(orm.em.getConnection(), tableNames);
           const fkDeleteRules = await collectForeignKeyDeleteRules(orm.em.getConnection(), tableNames);
-          fixups = applyRules(metadata, platform, implicitUniques, fkDeleteRules);
+          const expressionIndexes = await collectExpressionIndexes(orm.em.getConnection(), tableNames);
+          fixups = applyRules(metadata, platform, implicitUniques, fkDeleteRules, expressionIndexes);
         },
       });
 
diff --git a/server/src/db/email-case-index.ts b/server/src/db/email-case-index.ts
new file mode 100644
index 000000000..0b2330717
--- /dev/null
+++ b/server/src/db/email-case-index.ts
@@ -0,0 +1,38 @@
+import type { Connection } from '@mikro-orm/core';
+
+/**
+ * The case-insensitive unique index on users.email, when its migration had to
+ * leave it out.
+ *
+ * `Migration20200101042400_one_account_per_email_whatever_its_case` creates
+ * `idx_users_email_lower` unless two accounts already share an address in
+ * different case; then it records `email_case_index_pending` and the install
+ * boots without it. A migration runs once, so this runs on every boot while
+ * that flag is set: once the accounts were told apart it creates the index and
+ * clears the flag, until then it repeats the warning.
+ */
+
+const PENDING_KEY = 'email_case_index_pending';
+
+export async function ensureEmailCaseIndex(connection: Connection): Promise<void> {
+  const pending: unknown[] = await connection.execute(`SELECT value FROM app_settings WHERE key = ?`, [PENDING_KEY]);
+  if (pending.length === 0) return;
+
+  const duplicates: Array<{ ids: string }> = await connection.execute(
+    `SELECT group_concat(id, ', ') AS ids FROM (SELECT id, email FROM users ORDER BY id) GROUP BY lower(email) HAVING COUNT(*) > 1`,
+  );
+  if (duplicates.length > 0) {
+    console.warn(
+      `[DB] ${duplicates.length} email address(es) still belong to more than one account when case is ignored ` +
+        `(user ids ${duplicates.map((row) => row.ids).join('; ')}), so users.email has no case-insensitive unique ` +
+        'index yet. Give each of those accounts its own address under Admin > Users; the next start creates it.',
+    );
+    return;
+  }
+
+  await connection.transactional(async (trx) => {
+    await connection.execute('CREATE UNIQUE INDEX IF NOT EXISTS idx_users_email_lower ON users (lower(email))', [], 'run', trx);
+    await connection.execute('DELETE FROM app_settings WHERE key = ?', [PENDING_KEY], 'run', trx);
+  });
+  console.log('[DB] Every account has its own email address now; created the case-insensitive unique index on users.email');
+}
diff --git a/server/src/db/entities/Users.entity.ts b/server/src/db/entities/Users.entity.ts
index 572eacbd7..f28693097 100644
--- a/server/src/db/entities/Users.entity.ts
+++ b/server/src/db/entities/Users.entity.ts
@@ -207,6 +207,10 @@ export const UsersSchema = defineEntity({
     },
     { properties: ['email'] },
     { properties: ['username'] },
+    {
+      name: 'idx_users_email_lower',
+      expression: 'CREATE UNIQUE INDEX idx_users_email_lower ON users (lower(email))',
+    },
   ],
   properties: {
     id: p.integer().primary(),
diff --git a/server/src/db/migrations/Migration20200101042400_one_account_per_email_whatever_its_case.ts b/server/src/db/migrations/Migration20200101042400_one_account_per_email_whatever_its_case.ts
new file mode 100644
index 000000000..3a5678c60
--- /dev/null
+++ b/server/src/db/migrations/Migration20200101042400_one_account_per_email_whatever_its_case.ts
@@ -0,0 +1,45 @@
+import { Migration } from '@mikro-orm/migrations';
+
+/** One email address, lowered, that more than one account holds: the ids of those accounts. */
+interface DuplicateRow {
+  ids: string;
+}
+
+/**
+ * One account per email address, whatever its case.
+ *
+ * Login, password reset and the sign-up checks all match emails
+ * case-insensitively, but the only constraint is the column's own UNIQUE,
+ * which SQLite compares case-sensitively. Two accounts whose emails differ
+ * only in case (a legacy row, two sign-ups at once) left a login to resolve to
+ * whichever row came first. An index on lower(email) closes that in the
+ * database, and gives the case-insensitive lookups an index to use.
+ *
+ * An install that already holds such a pair must still boot, so the index is
+ * then left out, the pair is named in the log, and `app_settings` remembers
+ * that it is owed: `db/email-case-index.ts` creates it on the first boot after
+ * the accounts were told apart. The app-level checks keep guarding meanwhile.
+ */
+export class Migration20200101042400_one_account_per_email_whatever_its_case extends Migration {
+  override name = 'Migration20200101042400_one_account_per_email_whatever_its_case';
+
+  override async up(): Promise<void> {
+    const duplicates = (await this.execute(
+      `SELECT group_concat(id, ', ') AS ids FROM (SELECT id, email FROM users ORDER BY id) ` +
+        'GROUP BY lower(email) HAVING COUNT(*) > 1',
+    )) as DuplicateRow[];
+
+    if (duplicates.length > 0) {
+      console.warn(
+        `[migration] ${duplicates.length} email address(es) belong to more than one account when case is ignored ` +
+          `(user ids ${duplicates.map((row) => row.ids).join('; ')}). The case-insensitive unique ` +
+          'index on users.email is not created yet. Give each of those accounts its own address under ' +
+          'Admin > Users (or delete the one nobody uses); the next start creates the index.',
+      );
+      this.addSql(`INSERT OR REPLACE INTO app_settings (key, value) VALUES ('email_case_index_pending', 'true')`);
+      return;
+    }
+
+    this.addSql('CREATE UNIQUE INDEX IF NOT EXISTS idx_users_email_lower ON users (lower(email))');
+  }
+}
diff --git a/server/src/db/orm.ts b/server/src/db/orm.ts
index d811ec656..f10d29944 100644
--- a/server/src/db/orm.ts
+++ b/server/src/db/orm.ts
@@ -1,5 +1,6 @@
 import mikroOrmConfig from '../mikro-orm.config';
 import { registerReinitializeHook, runDemoSeed } from './database';
+import { ensureEmailCaseIndex } from './email-case-index';
 import { migrateToHead, NO_SAFETY_NET } from './legacy-baseline';
 import { preMigrateSnapshot } from './pre-migrate-snapshot';
 import { withRequestContext } from '../nest/database/request-context';
@@ -45,6 +46,8 @@ export async function runSchemaBootstrap(orm: AnyOrm, { snapshot = true }: { sna
   // run would replay the whole history over it (see legacy-baseline.ts). Every
   // other database just gets its pending migrations.
   await migrateToHead(connection, migrator, undefined, undefined, snapshot ? preMigrateSnapshot(connection) : NO_SAFETY_NET);
+  // The one index a migration may have had to defer (see email-case-index.ts).
+  await ensureEmailCaseIndex(connection);
 
   const defaultSeeder = orm.config.get('seeder').defaultSeeder;
   if (defaultSeeder) await orm.seeder.seedString(defaultSeeder);
diff --git a/server/src/db/repositories/Users.repository.ts b/server/src/db/repositories/Users.repository.ts
index 5b569766a..926ffac6c 100644
--- a/server/src/db/repositories/Users.repository.ts
+++ b/server/src/db/repositories/Users.repository.ts
@@ -2,6 +2,7 @@ import { Users } from '../entities/Users.entity';
 import { toRow, type AssertRowKeys } from './_shared/rows';
 import { coalesce, coalesceOverrideWhileSame, columnIncrementedBy, currentTimestamp, lower, lowerParam } from '../dialect/sql-functions';
 import { TrekRepository } from './_shared/trek-repository';
+import { UniqueConstraintViolationException } from '@mikro-orm/core';
 
 /**
  * The three third-party keys that belong to the instance rather than to a
@@ -239,6 +240,32 @@ export interface NewAdminUserRow {
   role: string;
 }
 
+/**
+ * A write the database refused because another account already holds that
+ * email address (in any case, `idx_users_email_lower`) or username. The
+ * services check first and answer 409; this is the same answer for the write
+ * that lost a race against another one, instead of a 500.
+ */
+export class UserIdentityTakenError extends Error {
+  constructor(readonly field: 'email' | 'username') {
+    super(`${field} already taken`);
+    this.name = 'UserIdentityTakenError';
+  }
+}
+
+/** Runs a users write, turning a unique violation on email or username into {@link UserIdentityTakenError}. */
+async function guardIdentity<T>(write: () => Promise<T>): Promise<T> {
+  try {
+    return await write();
+  } catch (err) {
+    if (err instanceof UniqueConstraintViolationException) {
+      if (/users\.email|idx_users_email/.test(err.message)) throw new UserIdentityTakenError('email');
+      if (/users\.username/.test(err.message)) throw new UserIdentityTakenError('username');
+    }
+    throw err;
+  }
+}
+
 /** AD11 (`admin.service.ts#updateUser`, inside `uow.transactional`) — the columns the legacy COALESCE-shaped UPDATE can touch; a key the caller omits is left unchanged, the same net effect as binding NULL into `COALESCE(?, column)`. */
 export interface AdminEditPatch {
   username?: string;
@@ -429,17 +456,19 @@ export class UsersRepository extends TrekRepository<Users> {
    * `created_at` the same way a legacy INSERT-then-reselect pair did.
    */
   async insertUser(row: NewUserRow): Promise<UserRow> {
-    const id = await this.insert({
-      username: row.username,
-      email: row.email,
-      password_hash: row.password_hash,
-      role: row.role,
-      first_seen_version: row.first_seen_version,
-      login_count: 0,
-      oidc_sub: row.oidc_sub ?? null,
-      oidc_issuer: row.oidc_issuer ?? null,
-      avatar: row.avatar ?? null,
-    });
+    const id = await guardIdentity(() =>
+      this.insert({
+        username: row.username,
+        email: row.email,
+        password_hash: row.password_hash,
+        role: row.role,
+        first_seen_version: row.first_seen_version,
+        login_count: 0,
+        oidc_sub: row.oidc_sub ?? null,
+        oidc_issuer: row.oidc_issuer ?? null,
+        avatar: row.avatar ?? null,
+      }),
+    );
     const inserted = await this.findById(id);
     if (!inserted) {
       throw new Error('insertUser: read-back after insert found no row');
@@ -968,7 +997,7 @@ export class UsersRepository extends TrekRepository<Users> {
    */
   async patchProfile(id: number, changes: UserProfilePatch): Promise<void> {
     const platform = this.getEntityManager().getPlatform();
-    await this.nativeUpdate({ id }, { ...changes, updated_at: currentTimestamp(platform) });
+    await guardIdentity(() => this.nativeUpdate({ id }, { ...changes, updated_at: currentTimestamp(platform) }));
   }
 
   // ---------------------------------------------------------------------
@@ -1533,12 +1562,14 @@ export class UsersRepository extends TrekRepository<Users> {
    * statement handled by {@link findAdminSummary}.
    */
   async insertAdminCreatedUser(row: NewAdminUserRow): Promise<number> {
-    return await this.insert({
-      username: row.username,
-      email: row.email,
-      password_hash: row.password_hash,
-      role: row.role,
-    });
+    return await guardIdentity(() =>
+      this.insert({
+        username: row.username,
+        email: row.email,
+        password_hash: row.password_hash,
+        role: row.role,
+      }),
+    );
   }
 
   /**
@@ -1584,7 +1615,7 @@ export class UsersRepository extends TrekRepository<Users> {
    */
   async applyAdminEdit(id: number, patch: AdminEditPatch): Promise<void> {
     const platform = this.getEntityManager().getPlatform();
-    await this.nativeUpdate({ id }, { ...patch, updated_at: currentTimestamp(platform) });
+    await guardIdentity(() => this.nativeUpdate({ id }, { ...patch, updated_at: currentTimestamp(platform) }));
   }
 
   /**
diff --git a/server/src/nest/admin/admin.service.ts b/server/src/nest/admin/admin.service.ts
index 29bee08bd..2043f917a 100644
--- a/server/src/nest/admin/admin.service.ts
+++ b/server/src/nest/admin/admin.service.ts
@@ -30,7 +30,7 @@ import { PermissionsService } from '../permissions/permissions.service';
 import { PERMISSION_ACTIONS } from '../permissions/permissions.service';
 import { NotificationsService } from '../notifications/notifications.service';
 import { Users } from '../../db/entities/Users.entity';
-import type { UsersRepository, AdminEditPatch } from '../../db/repositories/Users.repository';
+import { UserIdentityTakenError, type UsersRepository, type AdminEditPatch } from '../../db/repositories/Users.repository';
 import { AuditLog } from '../../db/entities/AuditLog.entity';
 import type { AuditLogRepository } from '../../db/repositories/AuditLog.repository';
 import { AppSettings } from '../../db/entities/AppSettings.entity';
@@ -99,6 +99,11 @@ const VERSION_FAILURE_TTL = 60_000;
  * and module-scoped pieces (compareVersions, isDocker, the version cache) live
  * in admin.helpers.ts.
  */
+/** The 409 the up-front checks give, for a write that lost the race to another one. */
+function identityTakenMessage(err: UserIdentityTakenError): string {
+  return err.field === 'email' ? 'Email already taken' : 'Username already taken';
+}
+
 @Injectable()
 export class AdminService {
   constructor(
@@ -175,9 +180,16 @@ export class AdminService {
 
     const passwordHash = bcrypt.hashSync(password, BCRYPT_COST);
 
-    const insertedId = await this.users.insertAdminCreatedUser({
-      username, email, password_hash: passwordHash, role: data.role || 'user',
-    });
+    let insertedId: number;
+    try {
+      insertedId = await this.users.insertAdminCreatedUser({
+        username, email, password_hash: passwordHash, role: data.role || 'user',
+      });
+    } catch (err) {
+      // Another write took the name or address between the check and the insert.
+      if (err instanceof UserIdentityTakenError) return { error: identityTakenMessage(err), status: 409 };
+      throw err;
+    }
 
     const user = await this.users.findAdminSummary(insertedId);
 
@@ -254,22 +266,27 @@ export class AdminService {
     // (R4): the users UPDATE, the mcp_tokens DELETE, the oauth_tokens revoke
     // and the push_subscriptions DELETE stay inside the SAME uow.transactional
     // call, never split across separate un-transacted repository calls.
-    await this.uow.transactional(async () => {
-      await this.users.applyAdminEdit(userId, patch);
+    try {
+      await this.uow.transactional(async () => {
+        await this.users.applyAdminEdit(userId, patch);
 
-      if (password) {
-        // The version bump only invalidates JWT cookies. These two stores carry
-        // their own credentials and are revoked separately, exactly as the
-        // self-service paths do it.
-        await this.mcpTokens.deleteAllForUser(userId);
-        try {
-          await this.oauthTokens.revokeAllForUser(userId);
-        } catch { /* very old installs predate oauth_tokens */ }
-        // Push devices outlive every session, so the intruder's browser would
-        // keep receiving this account's notifications. They go with the rest.
-        await this.pushSubscriptions.deleteAllForUser(userId);
-      }
-    });
+        if (password) {
+          // The version bump only invalidates JWT cookies. These two stores carry
+          // their own credentials and are revoked separately, exactly as the
+          // self-service paths do it.
+          await this.mcpTokens.deleteAllForUser(userId);
+          try {
+            await this.oauthTokens.revokeAllForUser(userId);
+          } catch { /* very old installs predate oauth_tokens */ }
+          // Push devices outlive every session, so the intruder's browser would
+          // keep receiving this account's notifications. They go with the rest.
+          await this.pushSubscriptions.deleteAllForUser(userId);
+        }
+      });
+    } catch (err) {
+      if (err instanceof UserIdentityTakenError) return { error: identityTakenMessage(err), status: 409 };
+      throw err;
+    }
 
     if (password) {
       try { revokeUserSessions(Number(id)); } catch { /* best-effort, same as elsewhere */ }
diff --git a/server/src/nest/auth/auth.service.ts b/server/src/nest/auth/auth.service.ts
index e7231e02a..92634f1f1 100644
--- a/server/src/nest/auth/auth.service.ts
+++ b/server/src/nest/auth/auth.service.ts
@@ -46,7 +46,7 @@ import { splitManagedKeys } from '../common/managed';
 import { emitUserDeleted } from '../../plugin-user-lifecycle';
 import { verifyJwtAndLoadUser } from './jwt-verify';
 import { User } from '../../types';
-import type { UserRow } from '../../db/repositories/Users.repository';
+import { UserIdentityTakenError, type UserRow } from '../../db/repositories/Users.repository';
 import { DEMO_EMAIL_PRIMARY, DEMO_PASS, isDemoEmail } from '../common/demo';
 import { avatarUrl } from '../common/avatarUrl';
 import { TripMembershipService } from '../trip-membership/trip-membership.service';
@@ -520,7 +520,12 @@ export class AuthService {
           auditDetails: { username, email, role },
         };
       });
-    } catch {
+    } catch (err) {
+      // The check above runs in the transaction, but a concurrent signup can
+      // still win the insert; the database's unique indexes refuse the second.
+      if (err instanceof UserIdentityTakenError) {
+        return { error: 'Registration failed. Please try different credentials.', status: 409 };
+      }
       return { error: 'Error creating user', status: 500 };
     }
   }
diff --git a/server/src/nest/auth/user-profile.service.ts b/server/src/nest/auth/user-profile.service.ts
index bd88302be..44d7df990 100644
--- a/server/src/nest/auth/user-profile.service.ts
+++ b/server/src/nest/auth/user-profile.service.ts
@@ -19,7 +19,7 @@ import { SEARCH_TEXT_FIELD_MASK } from '../maps/maps.helpers';
 import { AppSettings } from '../../db/entities/AppSettings.entity';
 import type { AppSettingsRepository } from '../../db/repositories/AppSettings.repository';
 import { Users } from '../../db/entities/Users.entity';
-import type { UsersRepository, UserApiKeyColumns, UserProfilePatch } from '../../db/repositories/Users.repository';
+import { UserIdentityTakenError, type UsersRepository, type UserApiKeyColumns, type UserProfilePatch } from '../../db/repositories/Users.repository';
 
 /** A key check is a single request to a provider; one that hangs must not hold the settings page. */
 const KEY_CHECK_TIMEOUT_MS = 10_000;
@@ -233,10 +233,18 @@ export class UserProfileService {
     const changedKeys = keyLocked ? [] : await this.changedKeyNames(body, current, isAdmin, blocked);
 
     if (Object.keys(changes).length > 0) {
-      await this.uow.transactional(async () => {
-        await this.usersRepo.patchProfile(userId, changes);
-        if (!keyLocked) await this.mirrorInstanceKeys(body, isAdmin);
-      });
+      try {
+        await this.uow.transactional(async () => {
+          await this.usersRepo.patchProfile(userId, changes);
+          if (!keyLocked) await this.mirrorInstanceKeys(body, isAdmin);
+        });
+      } catch (err) {
+        // Another write took the name or address between the check above and this one.
+        if (err instanceof UserIdentityTakenError) {
+          return { error: err.field === 'email' ? 'Email already taken' : 'Username already taken', status: 409 };
+        }
+        throw err;
+      }
     }
 
     const updated = await this.usersRepo.findProfileWithKeys(userId);
diff --git a/server/tests/unit/db/email-case-index.test.ts b/server/tests/unit/db/email-case-index.test.ts
new file mode 100644
index 000000000..8826e4aaa
--- /dev/null
+++ b/server/tests/unit/db/email-case-index.test.ts
@@ -0,0 +1,99 @@
+/**
+ * `Migration20200101042400_one_account_per_email_whatever_its_case` and the
+ * boot step that finishes its job (`src/db/email-case-index.ts`): one account
+ * per email address whatever its case, without ever failing the boot of an
+ * install that already holds such a pair.
+ */
+import { ensureEmailCaseIndex } from '../../../src/db/email-case-index';
+import { createMigrationOrm, migrateTo, pendingNames, rawExec, rawQuery } from '../../helpers/migration-step';
+import type { MikroORM } from '@mikro-orm/sqlite';
+
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+const TARGET = 'Migration20200101042400_one_account_per_email_whatever_its_case';
+
+let orm: MikroORM | null = null;
+
+async function ormBeforeTarget(): Promise<MikroORM> {
+  orm = await createMigrationOrm();
+  const names = await pendingNames(orm);
+  const idx = names.indexOf(TARGET);
+  expect(idx).toBeGreaterThan(0);
+  await migrateTo(orm, names[idx - 1]);
+  return orm;
+}
+
+async function addUser(db: MikroORM, id: number, email: string): Promise<void> {
+  await rawExec(db, 'INSERT INTO users (id, username, email, password_hash) VALUES (?, ?, ?, ?)', [id, `user${id}`, email, 'x']);
+}
+
+const hasIndex = async (db: MikroORM) =>
+  (await rawQuery(db, `SELECT name FROM sqlite_master WHERE type = 'index' AND name = 'idx_users_email_lower'`)).length === 1;
+const pendingFlag = async (db: MikroORM) =>
+  rawQuery<{ value: string }>(db, `SELECT value FROM app_settings WHERE key = 'email_case_index_pending'`);
+
+afterEach(async () => {
+  vi.restoreAllMocks();
+  await orm?.close(true);
+  orm = null;
+});
+
+describe('one account per email address, whatever its case', () => {
+  it('EMAILCI-001: without case duplicates the index is created and refuses a second account in other case', async () => {
+    const db = await ormBeforeTarget();
+    await addUser(db, 1, 'Anna@Example.test');
+    await addUser(db, 2, 'bob@example.test');
+
+    await migrateTo(db, TARGET);
+
+    expect(await hasIndex(db)).toBe(true);
+    expect(await pendingFlag(db)).toEqual([]);
+    await expect(addUser(db, 3, 'anna@example.TEST')).rejects.toThrow(/UNIQUE constraint failed/);
+    // Mixed case itself is fine: only a second account for the same address is not.
+    expect(await rawQuery(db, 'SELECT email FROM users ORDER BY id')).toEqual([
+      { email: 'Anna@Example.test' },
+      { email: 'bob@example.test' },
+    ]);
+  }, 30000);
+
+  it('EMAILCI-002: with case duplicates the boot goes on, names the accounts, and the index waits', async () => {
+    const db = await ormBeforeTarget();
+    await addUser(db, 1, 'anna@example.test');
+    await addUser(db, 2, 'Anna@Example.test');
+    await addUser(db, 3, 'carl@example.test');
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+    await expect(migrateTo(db, TARGET)).resolves.toBeUndefined();
+
+    expect(await hasIndex(db)).toBe(false);
+    expect(await pendingFlag(db)).toEqual([{ value: 'true' }]);
+    const message = String(warn.mock.calls[0]?.[0]);
+    expect(message).toContain('1 email address(es) belong to more than one account');
+    expect(message).toContain('user ids 1, 2');
+    expect(message).not.toContain('anna@example.test');
+  }, 30000);
+
+  it('EMAILCI-003: the boot step repeats the warning while the pair remains, then creates the index once it is gone', async () => {
+    const db = await ormBeforeTarget();
+    await addUser(db, 1, 'anna@example.test');
+    await addUser(db, 2, 'ANNA@example.test');
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    vi.spyOn(console, 'log').mockImplementation(() => {});
+    await migrateTo(db, TARGET);
+    warn.mockClear();
+
+    await ensureEmailCaseIndex(db.em.getConnection());
+    expect(await hasIndex(db)).toBe(false);
+    expect(String(warn.mock.calls[0]?.[0])).toContain('still belong to more than one account');
+
+    await rawExec(db, `UPDATE users SET email = 'anna.work@example.test' WHERE id = 2`);
+    await ensureEmailCaseIndex(db.em.getConnection());
+    expect(await hasIndex(db)).toBe(true);
+    expect(await pendingFlag(db)).toEqual([]);
+
+    // Nothing owed any more: a later boot does nothing.
+    warn.mockClear();
+    await ensureEmailCaseIndex(db.em.getConnection());
+    expect(warn).not.toHaveBeenCalled();
+  }, 30000);
+});
diff --git a/server/tests/unit/db/generate-entities.test.ts b/server/tests/unit/db/generate-entities.test.ts
index f9bf88b84..92464a423 100644
--- a/server/tests/unit/db/generate-entities.test.ts
+++ b/server/tests/unit/db/generate-entities.test.ts
@@ -34,6 +34,7 @@ import {
   RULE11_referencedColumns,
   RULE12_fixGarbledCheckExpressions,
   RULE13_pinDeleteRuleDrift,
+  RULE14_addExpressionIndexes,
   RULE_normalizeLiteralDefaults,
   applyTextPasses,
   checkEntities,
@@ -694,6 +695,33 @@ describe('collectForeignKeyDeleteRules', () => {
   });
 });
 
+describe('RULE14_addExpressionIndexes', () => {
+  const sql = 'CREATE UNIQUE INDEX idx_users_email_lower ON users (lower(email))';
+
+  it('RULE14-001: a unique expression index becomes a named uniques: entry carrying its statement', () => {
+    const meta = fixtureMeta('Users', 'users', [fixtureProp({ name: 'email', primary: false })]);
+    RULE14_addExpressionIndexes([meta], new Map([['users', [{ name: 'idx_users_email_lower', unique: true, sql }]]]));
+    expect(meta.uniques).toEqual([{ name: 'idx_users_email_lower', expression: sql }]);
+    expect(meta.indexes).toEqual([]);
+  });
+
+  it('RULE14-002: a plain expression index goes to indexes:, and an introspected entry of the same name is replaced', () => {
+    const meta = fixtureMeta('X', 'x', [fixtureProp({ name: 'name', primary: false })]);
+    meta.indexes.push({ name: 'idx_x_name_lower', properties: [] });
+    const plain = 'CREATE INDEX idx_x_name_lower ON x (lower(name))';
+    RULE14_addExpressionIndexes([meta], new Map([['x', [{ name: 'idx_x_name_lower', unique: false, sql: plain }]]]));
+    expect(meta.indexes).toEqual([{ name: 'idx_x_name_lower', expression: plain }]);
+    expect(meta.uniques).toEqual([]);
+  });
+
+  it('RULE14-003: a table without expression indexes is left alone', () => {
+    const meta = fixtureMeta('X', 'x', [fixtureProp({ name: 'name', primary: false })]);
+    RULE14_addExpressionIndexes([meta], new Map([['users', [{ name: 'idx_users_email_lower', unique: true, sql }]]]));
+    expect(meta.uniques).toEqual([]);
+    expect(meta.indexes).toEqual([]);
+  });
+});
+
 describe('RULE9_addImplicitUniqueConstraints', () => {
   it('RULE9-001: a table with a matching implicit-unique entry gets a uniques: block referencing the columns as properties — a plain (non-FK) column keeps its own name', () => {
     const userId = fixtureProp({ name: 'user_id', primary: false });
diff --git a/server/tests/unit/db/legacy-baseline.test.ts b/server/tests/unit/db/legacy-baseline.test.ts
index 5a55a818d..708505212 100644
--- a/server/tests/unit/db/legacy-baseline.test.ts
+++ b/server/tests/unit/db/legacy-baseline.test.ts
@@ -35,6 +35,7 @@ const UNNUMBERED = [
   'Migration20200101042100_a_trip_reminder_remembers_it_was_sent',
   'Migration20200101042200_an_index_on_every_foreign_key',
   'Migration20200101042300_every_tour_type_the_contract_names',
+  'Migration20200101042400_one_account_per_email_whatever_its_case',
 ];
 // The last step the positional runner ever had; planLegacyBaseline refuses a
 // schema_version past it.
diff --git a/server/tests/unit/db/repositories/Users.repository.test.ts b/server/tests/unit/db/repositories/Users.repository.test.ts
index 834169cb1..b8e4685d3 100644
--- a/server/tests/unit/db/repositories/Users.repository.test.ts
+++ b/server/tests/unit/db/repositories/Users.repository.test.ts
@@ -5,6 +5,7 @@ import { createTestOrm, type TestOrm } from '../../../helpers/test-orm';
 import { createUser, createAdmin, type TestUser } from '../../../helpers/factories';
 import { Users } from '../../../../src/db/entities/Users.entity';
 import type { UsersRepository } from '../../../../src/db/repositories/Users.repository';
+import { UserIdentityTakenError } from '../../../../src/db/repositories/Users.repository';
 
 const testDb = createSnapshotTestDb();
 let t: TestOrm;
@@ -1202,3 +1203,25 @@ describe('UsersRepository — Immich settings write (IM4/IM5)', () => {
     expect(read(user.id)).toEqual({ immich_url: null, immich_api_key: null, immich_allow_insecure_tls: 0 });
   });
 });
+
+describe('a users write the unique indexes refuse', () => {
+  const row = (username: string, email: string) => ({
+    username, email, password_hash: 'hashed', role: 'user', first_seen_version: '1.0.0',
+  });
+
+  it('USERSREPO-IDENT-001: an email another account holds in different case is refused as the email', async () => {
+    createUser(testDb, { username: 'anna', email: 'anna@example.com' });
+    const refused = users.insertUser(row('anna2', 'ANNA@Example.com'));
+    await expect(refused).rejects.toBeInstanceOf(UserIdentityTakenError);
+    await expect(refused).rejects.toMatchObject({ field: 'email' });
+  });
+
+  it('USERSREPO-IDENT-002: a taken username is refused as the username, on insert and on update', async () => {
+    createUser(testDb, { username: 'anna', email: 'anna@example.com' });
+    const { user } = createUser(testDb, { username: 'carl', email: 'carl@example.com' });
+    await expect(users.insertAdminCreatedUser({ username: 'anna', email: 'new@example.com', password_hash: 'x', role: 'user' }))
+      .rejects.toMatchObject({ field: 'username' });
+    await expect(users.patchProfile(user.id, { email: 'Anna@example.com' })).rejects.toMatchObject({ field: 'email' });
+    await expect(users.applyAdminEdit(user.id, { username: 'anna' })).rejects.toMatchObject({ field: 'username' });
+  });
+});
diff --git a/server/tests/unit/nest/admin.service.test.ts b/server/tests/unit/nest/admin.service.test.ts
index 6e3e40ab3..d3a099918 100644
--- a/server/tests/unit/nest/admin.service.test.ts
+++ b/server/tests/unit/nest/admin.service.test.ts
@@ -10,6 +10,7 @@
  * (it replaced ADMIN-BR-001 when the old admin bridge died with the cron move).
  */
 import { describe, it, expect, vi, beforeAll, beforeEach, afterEach, afterAll } from 'vitest';
+import { UsersRepository } from '../../../src/db/repositories/Users.repository';
 import { ADDON_IDS, MCP_GATED_ADDON_IDS } from '../../../src/addons';
 
 // ── DB setup ──────────────────────────────────────────────────────────────────
@@ -964,8 +965,8 @@ describe('updateUser — password-reset transaction boundary (AD11/12/13)', () =
   });
 });
 
-describe('createUser — the pre-existing uniqueness-check TOCTOU window (AD2-4, R4 "report, don\'t fix")', () => {
-  it('ADMIN-SVC-094 — a race between the email pre-check and the INSERT is not caught by a transaction; the row-level UNIQUE constraint is what actually stops the duplicate, surfacing as a thrown error rather than a clean 409', async () => {
+describe('createUser: the uniqueness-check TOCTOU window (AD2-4)', () => {
+  it('ADMIN-SVC-094: a race between the email pre-check and the INSERT is stopped by the row-level UNIQUE constraint and answered with the same clean 409', async () => {
     const { user: existing } = createUser(testDb, { email: 'race@test.example.com' });
 
     // Simulates the concurrent-request race AD2-4's non-transactional shape
@@ -978,15 +979,13 @@ describe('createUser — the pre-existing uniqueness-check TOCTOU window (AD2-4,
     const spy = vi.spyOn(usersRepo, 'findIdByEmailCI').mockResolvedValueOnce(null);
 
     // The pre-check said "clear," but `users.email` carries its own UNIQUE
-    // index (`Migration20200101000000_baseline_schema.ts`) — that's what
-    // actually stops the duplicate, at the SQL level, not this service's own
-    // TOCTOU-vulnerable pre-check. Because nothing here catches that
-    // failure, it propagates as an unhandled rejection rather than the
-    // clean `{ error: 'Email already taken', status: 409 }` a request that
-    // lost the race with more lead time would have gotten.
+    // index (`Migration20200101000000_baseline_schema.ts`), and that is what
+    // actually stops the duplicate, at the SQL level. The repository turns
+    // the refusal into UserIdentityTakenError, so the request that lost the
+    // race gets the same answer as one that lost it with more lead time.
     await expect(
       svcCreateUser({ username: 'raceuser', email: existing.email, password: 'ValidPass1!' }),
-    ).rejects.toThrow();
+    ).resolves.toEqual({ error: 'Email already taken', status: 409 });
 
     spy.mockRestore();
   });
@@ -1002,3 +1001,27 @@ describe('createUser — the pre-existing uniqueness-check TOCTOU window (AD2-4,
     expect(existing.id).toBeGreaterThan(0);
   });
 });
+
+describe('a create or edit that loses the race for an email or username', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('ADMIN-IDENT-001: createUser answers the same 409 when the database refuses what the check missed', async () => {
+    createUser(testDb, { username: 'anna', email: 'anna@example.com' });
+    // The other write lands between the check and the insert.
+    vi.spyOn(UsersRepository.prototype, 'findIdByEmailCI').mockResolvedValue(null);
+    expect(await svcCreateUser({ username: 'anna2', email: 'ANNA@example.com', password: 'Secure123!' })).toEqual({
+      error: 'Email already taken',
+      status: 409,
+    });
+  });
+
+  it('ADMIN-IDENT-002: updateUser does the same, and changes nothing', async () => {
+    createUser(testDb, { username: 'anna', email: 'anna@example.com' });
+    const { user } = createUser(testDb, { username: 'carl', email: 'carl@example.com' });
+    vi.spyOn(UsersRepository.prototype, 'findIdByEmailCI').mockResolvedValue(null);
+    expect(await updateUser(String(user.id), { email: 'Anna@Example.com' })).toEqual({ error: 'Email already taken', status: 409 });
+    expect(testDb.prepare('SELECT email FROM users WHERE id = ?').get(user.id)).toEqual({ email: 'carl@example.com' });
+  });
+});
diff --git a/server/tests/unit/nest/auth.service.test.ts b/server/tests/unit/nest/auth.service.test.ts
index 69e4a7e54..ee59ada4d 100644
--- a/server/tests/unit/nest/auth.service.test.ts
+++ b/server/tests/unit/nest/auth.service.test.ts
@@ -60,6 +60,7 @@ vi.mock('../../../src/mcp/sessionManager', () => ({ revokeUserSessions: vi.fn()
 
 import { db as testDb } from '../../../src/db/database';
 import { describe, it, expect, beforeAll, beforeEach, afterEach, afterAll, vi } from 'vitest';
+import { UsersRepository } from '../../../src/db/repositories/Users.repository';
 import { resetTestDb } from '../../helpers/test-db';
 import { createUser, createAdmin, createInviteToken, createTrip, createPlace, createReservation } from '../../helpers/factories';
 import { AuthService } from '../../../src/nest/auth/auth.service';
@@ -1251,3 +1252,18 @@ describe('generateToken remember claim (#1927)', () => {
     expect(decoded.exp - decoded.iat).toBe(86400);
   });
 });
+
+describe('registerUser loses the race for an email', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('AUTH-IDENT-001: a signup the database refuses after the check passed answers the same 409, not a 500', async () => {
+    createUser(testDb, { username: 'anna', email: 'anna@x.com' });
+    vi.spyOn(UsersRepository.prototype, 'findIdByEmailOrUsernameCI').mockResolvedValue(null);
+    expect(await svc.registerUser({ username: 'anna2', email: 'ANNA@x.com', password: 'Secure123!' })).toEqual({
+      error: 'Registration failed. Please try different credentials.',
+      status: 409,
+    });
+  });
+});
diff --git a/server/tests/unit/nest/user-profile.service.test.ts b/server/tests/unit/nest/user-profile.service.test.ts
index 933ab2e36..30474912e 100644
--- a/server/tests/unit/nest/user-profile.service.test.ts
+++ b/server/tests/unit/nest/user-profile.service.test.ts
@@ -38,6 +38,7 @@ import { createUser, createAdmin } from '../../helpers/factories';
 import fs from 'node:fs';
 import path from 'node:path';
 import { UserProfileService } from '../../../src/nest/auth/user-profile.service';
+import { UsersRepository } from '../../../src/db/repositories/Users.repository';
 import { makeStorageFixture } from '../../helpers/storage-fixture';
 import { createTestUnitOfWork, createTestAppSettingsRepo, createTestUsersRepo } from '../../helpers/test-uow';
 import { SEARCH_TEXT_FIELD_MASK } from '../../../src/nest/maps/maps.helpers';
@@ -505,3 +506,17 @@ describe('changedKeys', () => {
     }
   });
 });
+
+describe('updateSettings loses the race for an email', () => {
+  it('AUTH-IDENT-002: the database refusing what the check missed answers the same 409', async () => {
+    createUser(testDb, { email: 'anna@example.com' });
+    const { user } = createUser(testDb, { email: 'carl@example.com' });
+    const check = vi.spyOn(UsersRepository.prototype, 'findIdByEmailCI').mockResolvedValue(null);
+    try {
+      const result = await profile.updateSettings(user.id, { email: 'ANNA@example.com' });
+      expect(result).toEqual({ error: 'Email already taken', status: 409 });
+    } finally {
+      check.mockRestore();
+    }
+  });
+});
diff --git a/wiki/Admin-Users-and-Invites.md b/wiki/Admin-Users-and-Invites.md
index 7f7eea944..834908ebb 100644
--- a/wiki/Admin-Users-and-Invites.md
+++ b/wiki/Admin-Users-and-Invites.md
@@ -32,6 +32,10 @@ Click the pencil icon (**Edit User**) on any row to open the edit dialog. You ca
 
 Click **Save** to apply changes.
 
+An email address belongs to one account only, and case does not count: `Anna@Example.com` and `anna@example.com` are the same address. Saving an address another account already holds answers **Email already taken**, and creating such an account does the same.
+
+If an install from before 4.4 already holds two accounts whose addresses differ only in case, the update still starts, and the log names their user ids: `email address(es) belong to more than one account when case is ignored`. Give each of those accounts its own address here (or delete the one nobody uses). The next start then adds the database rule that keeps it that way.
+
 Below the fields sits **Reset passkeys**, which removes every passkey that user has registered: the recovery path when someone loses their authenticator. It asks for confirmation in a dialog, reports how many passkeys it removed, and takes effect immediately rather than on **Save**. The user can still sign in with their password. See [Passkeys](Passkeys).
 
 ### Delete a user