diff --git a/changelog.d/7193.misc b/changelog.d/7193.misc
new file mode 100644
index 000000000000..383a738e6497
--- /dev/null
+++ b/changelog.d/7193.misc
@@ -0,0 +1 @@
+Add a background database update job to clear out duplicate `device_lists_outbound_pokes`.
diff --git a/synapse/storage/data_stores/main/devices.py b/synapse/storage/data_stores/main/devices.py
index dd3561e9b20d..b2100bb43aa8 100644
--- a/synapse/storage/data_stores/main/devices.py
+++ b/synapse/storage/data_stores/main/devices.py
@@ -32,7 +32,11 @@
 )
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
-from synapse.storage.database import Database, LoggingTransaction
+from synapse.storage.database import (
+    Database,
+    LoggingTransaction,
+    make_tuple_comparison_clause,
+)
 from synapse.types import Collection, get_verify_key_from_cross_signing_key
 from synapse.util.caches.descriptors import (
     Cache,
@@ -49,6 +53,8 @@
     "drop_device_list_streams_non_unique_indexes"
 )
 
+BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES = "remove_dup_outbound_pokes"
+
 
 class DeviceWorkerStore(SQLBaseStore):
     def get_device(self, user_id, device_id):
@@ -714,6 +720,11 @@ def __init__(self, database: Database, db_conn, hs):
             self._drop_device_list_streams_non_unique_indexes,
         )
 
+        # clear out duplicate device list outbound pokes
+        self.db.updates.register_background_update_handler(
+            BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES, self._remove_duplicate_outbound_pokes,
+        )
+
     @defer.inlineCallbacks
     def _drop_device_list_streams_non_unique_indexes(self, progress, batch_size):
         def f(conn):
@@ -728,6 +739,66 @@ def f(conn):
         )
         return 1
 
+    async def _remove_duplicate_outbound_pokes(self, progress, batch_size):
+        # for some reason, we have accumulated duplicate entries in
+        # device_lists_outbound_pokes, which makes prune_outbound_device_list_pokes less
+        # efficient.
+        #
+        # For each duplicate, we delete all the existing rows and put one back.
+
+        KEY_COLS = ["stream_id", "destination", "user_id", "device_id"]
+        last_row = progress.get(
+            "last_row",
+            {"stream_id": 0, "destination": "", "user_id": "", "device_id": "",},
+        )
+
+        def _txn(txn):
+            clause, args = make_tuple_comparison_clause(
+                self.db.engine, [(x, last_row[x]) for x in KEY_COLS]
+            )
+            sql = """
+                SELECT stream_id, destination, user_id, device_id, MAX(ts) AS ts
+                FROM device_lists_outbound_pokes
+                WHERE %s
+                GROUP BY %s
+                HAVING count(*) > 1
+                ORDER BY %s
+                LIMIT ?
+                """ % (
+                clause,  # WHERE
+                ",".join(KEY_COLS),  # GROUP BY
+                ",".join(KEY_COLS),  # ORDER BY
+            )
+            txn.execute(sql, args + [batch_size])
+            rows = self.db.cursor_to_dict(txn)
+
+            row = None
+            for row in rows:
+                self.db.simple_delete_txn(
+                    txn, "device_lists_outbound_pokes", {x: row[x] for x in KEY_COLS},
+                )
+
+                row["sent"] = False
+                self.db.simple_insert_txn(
+                    txn, "device_lists_outbound_pokes", row,
+                )
+
+            if row:
+                self.db.updates._background_update_progress_txn(
+                    txn, BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES, {"last_row": row},
+                )
+
+            return len(rows)
+
+        rows = await self.db.runInteraction(BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES, _txn)
+
+        if not rows:
+            await self.db.updates._end_background_update(
+                BG_UPDATE_REMOVE_DUP_OUTBOUND_POKES
+            )
+
+        return rows
+
 
 class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
     def __init__(self, database: Database, db_conn, hs):
diff --git a/synapse/storage/data_stores/main/schema/delta/58/02remove_dup_outbound_pokes.sql b/synapse/storage/data_stores/main/schema/delta/58/02remove_dup_outbound_pokes.sql
new file mode 100644
index 000000000000..f1abef5b175a
--- /dev/null
+++ b/synapse/storage/data_stores/main/schema/delta/58/02remove_dup_outbound_pokes.sql
@@ -0,0 +1,22 @@
+/* Copyright 2020 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.
+ */
+
+ /* for some reason, we have accumulated duplicate entries in
+  * device_lists_outbound_pokes, which makes prune_outbound_device_list_pokes less
+  * efficient.
+  */
+
+INSERT INTO background_updates (ordering, update_name, progress_json)
+    VALUES (5800, "remove_dup_outbound_pokes", "{}");