diff --git a/changelog.d/14669.bugfix b/changelog.d/14669.bugfix
new file mode 100644
index 000000000..bea316b06
--- /dev/null
+++ b/changelog.d/14669.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.70.0 which could cause spurious `UNIQUE constraint failed` errors in the `rotate_notifs` background job.
diff --git a/synapse/storage/databases/main/event_push_actions.py b/synapse/storage/databases/main/event_push_actions.py
index 7ebe34f77..3a0c370fd 100644
--- a/synapse/storage/databases/main/event_push_actions.py
+++ b/synapse/storage/databases/main/event_push_actions.py
@@ -274,15 +274,6 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
                 self._clear_old_push_actions_staging, 30 * 60 * 1000
             )
 
-        self.db_pool.updates.register_background_index_update(
-            "event_push_summary_unique_index",
-            index_name="event_push_summary_unique_index",
-            table="event_push_summary",
-            columns=["user_id", "room_id"],
-            unique=True,
-            replaces_index="event_push_summary_user_rm",
-        )
-
         self.db_pool.updates.register_background_index_update(
             "event_push_summary_unique_index2",
             index_name="event_push_summary_unique_index2",
diff --git a/synapse/storage/schema/main/delta/73/23_fix_thread_index.sql b/synapse/storage/schema/main/delta/73/23_fix_thread_index.sql
new file mode 100644
index 000000000..ec519ceeb
--- /dev/null
+++ b/synapse/storage/schema/main/delta/73/23_fix_thread_index.sql
@@ -0,0 +1,33 @@
+/* Copyright 2022 The Matrix.org Foundation C.I.C
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- If a Synapse deployment made a large jump in versions (from < 1.62.0 to >= 1.70.0)
+-- in a single upgrade then it might be possible for the event_push_summary_unique_index
+-- to be created in the background from delta 71/02event_push_summary_unique.sql after
+-- delta 73/06thread_notifications_thread_id_idx.sql is executed, causing it to
+-- not drop the event_push_summary_unique_index index.
+--
+-- See https://github.com/matrix-org/synapse/issues/14641
+
+-- Stop the index from being scheduled for creation in the background.
+DELETE FROM background_updates WHERE update_name = 'event_push_summary_unique_index';
+
+-- The above background job also replaces another index, so ensure that side-effect
+-- is applied.
+DROP INDEX IF EXISTS event_push_summary_user_rm;
+
+-- Fix deployments which ran the 73/06thread_notifications_thread_id_idx.sql delta
+-- before the event_push_summary_unique_index background job was run.
+DROP INDEX IF EXISTS event_push_summary_unique_index;