diff --git a/packages/backend/prisma/migrations/20261020090000_users_email_normalised/migration.sql b/packages/backend/prisma/migrations/20261020090000_users_email_normalised/migration.sql
new file mode 100644
index 000000000..8e41da841
--- /dev/null
+++ b/packages/backend/prisma/migrations/20261020090000_users_email_normalised/migration.sql
@@ -0,0 +1,33 @@
+-- One address, one account, enforced by the database.
+--
+-- The application writes addresses trimmed and lower-cased and looks them up
+-- ignoring case (common/email.util.ts), and 20261019120000_normalize_user_emails
+-- rewrote the existing rows. Accounts that only differed in case have been
+-- merged by hand since, so every stored address is now normalised.
+--
+-- This check keeps it that way: together with the existing unique index on
+-- "email", an address that is stored normalised can only exist once in any
+-- case. A write path that forgets to normalise fails loudly here instead of
+-- silently creating a second account.
+--
+-- A CHECK constraint rather than a unique index on lower("email"): Prisma
+-- cannot describe expression indexes and would try to drop one on the next
+-- `migrate dev`, while it leaves check constraints alone.
+--
+-- Only added when every row already satisfies it. A self-hosted install can
+-- still hold case-variant duplicates that the previous migration left alone
+-- on purpose; a failing migration would keep the app from starting, so there
+-- it is skipped with a notice. Merge the rows listed by
+-- scripts/report-duplicate-emails.sql, then add the constraint by hand with
+-- the ALTER TABLE below.
+DO $$
+BEGIN
+  IF EXISTS (SELECT 1 FROM "users" WHERE "email" <> lower(btrim("email"))) THEN
+    RAISE NOTICE 'users_email_normalised_check not added: some addresses are not normalised (see scripts/report-duplicate-emails.sql)';
+  ELSIF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname = 'users_email_normalised_check') THEN
+    ALTER TABLE "users"
+      ADD CONSTRAINT "users_email_normalised_check"
+      CHECK ("email" = lower(btrim("email")));
+  END IF;
+END
+$$;