ping-v10.1 room appears to cause minor race conditions

[turt2live]

ss_sync  | 22:00:03 WRN parseRoomsResponse: prepending state events to timeline after gappy poll prependStateEvents=65 room_id=!ping-v10.1:maunium.net
ss_sync  | 22:00:03 WRN Accumulator.filterAndParseTimelineEvents: seen the same event ID twice, ignoring event_id=$Y3-zTlg4P3O_8kR0UxEZq3eKJ6DUno6rKmLqE_iMRek room_id=!ping-v10.1:maunium.net

[DMRobertson]

This is a problem with the upstream homeserver sending us the same event more than once in the timeline.

I would suspect Synapse is doing something silly:

• Duplicate m.room.create event in the timeline in the response to /sync synapse#1995
• Own member event is duplicated in timeline when joining a room synapse#9768
• Synapse is sending events to appservices multiple times synapse#11447
• An event was sent twice down /sync (and down application service txns?) consistently across clients; ghost notifications synapse#14539