Handle multiple rows device inbox (#17362)

Fix bug where we don't get new to-device from remote if they resent a
message we've already persisted and have recorded in the DB twice.

`device_federation_inbox` table doesn't have a unique index, and so we
can race and store an entry in there twice. If we do so then
`simple_select_one_txn` will throw an error due to the query returning
more than one row. We should add an unique index, but it doesn't really
matter so lets just handle the case of multiple rows correctly for now.

changelog.d/17362.bugfix

@@ -0,0 +1 @@
+Fix rare race which causes no new to-device messages to be received from remote server.

synapse/storage/databases/main/deviceinbox.py

@@ -825,14 +825,13 @@ def add_messages_txn(
             # Check if we've already inserted a matching message_id for that
             # origin. This can happen if the origin doesn't receive our
             # acknowledgement from the first time we received the message.
-            already_inserted = self.db_pool.simple_select_one_txn(
+            already_inserted = self.db_pool.simple_select_list_txn(
                 txn,
                 table="device_federation_inbox",
                 keyvalues={"origin": origin, "message_id": message_id},
                 retcols=("message_id",),
-                allow_none=True,
             )
-            if already_inserted is not None:
+            if already_inserted:
                 return
 
             # Add an entry for this message_id so that we know we've processed