diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index 9c9cf46e7f..0d18f6d869 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -623,7 +623,9 @@ class RegistrationStore(RegistrationWorkerStore,
         Removes the given user to the table of users who need to be parted from all the
         rooms they're in, effectively marking that user as fully deactivated.
         """
-        return self._simple_delete_one(
+        # XXX: This should be simple_delete_one but we failed to put a unique index on
+        # the table, so somehow duplicate entries have ended up in it.
+        return self._simple_delete(
             "users_pending_deactivation",
             keyvalues={
                 "user_id": user_id,