diff --git a/migration/1781016011873-AddUserDataMailLowerUniqueIndex.js b/migration/1781016011873-AddUserDataMailLowerUniqueIndex.js
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+// Replace the non-unique LOWER(mail) index (added in #3855) with a UNIQUE one to permanently
+// prevent duplicate accounts for the same e-mail address.
+//
+// GATING: this migration MUST NOT be merged/deployed until all existing case-insensitive
+// duplicate mails have been resolved (merged or nulled). Otherwise `CREATE UNIQUE INDEX` fails.
+// Pre-check (must return zero rows):
+//   SELECT LOWER(mail), array_agg(id), count(*) FROM user_data WHERE mail IS NOT NULL
+//   GROUP BY LOWER(mail) HAVING count(*) > 1;
+//
+// PREDICATE: the partial index intentionally mirrors the dedup set used by
+// `getUsersByMail(onlyValidUser)` and excludes 'Merged' and 'Blocked' rows. Merged slaves retain
+// their mail (mergeUserData does not null it), and blocked duplicates may keep theirs for audit;
+// a bare `WHERE mail IS NOT NULL` predicate would break on historical merges and on every future
+// merge. Scoping to the active set keeps uniqueness consistent with the lookup that enforces it.
+
+/**
+ * @typedef {import('typeorm').MigrationInterface} MigrationInterface
+ * @typedef {import('typeorm').QueryRunner} QueryRunner
+ */
+
+module.exports = class AddUserDataMailLowerUniqueIndex1781016011873 {
+  name = 'AddUserDataMailLowerUniqueIndex1781016011873';
+
+  async up(queryRunner) {
+    await queryRunner.query(`DROP INDEX "public"."IDX_user_data_mail_lower"`);
+    await queryRunner.query(
+      `CREATE UNIQUE INDEX "IDX_user_data_mail_lower" ON "user_data" (LOWER("mail")) WHERE "mail" IS NOT NULL AND "status" IN ('Active', 'NA', 'KycOnly', 'Deactivated')`,
+    );
+  }
+
+  async down(queryRunner) {
+    await queryRunner.query(`DROP INDEX "public"."IDX_user_data_mail_lower"`);
+    await queryRunner.query(`CREATE INDEX "IDX_user_data_mail_lower" ON "user_data" (LOWER("mail"))`);
+  }
+};