Event handlers for state events are sometimes called twice

[gingershaped]

While developing a bot using the matrix-rust-sdk, I have observed that event handlers for state events (in my case OriginalRoomMemberEvent) are occasionally called twice when a state event arrives. Client::handle_sync_timeline_events always calls the handler, but sometimes Client::handle_sync_state_events also calls it. This behavior makes it difficult to write a bot that needs to respond to state events, unless the event handlers can be made idempotent.

[zecakeh]

To summarize what was discussed in the Matrix Rust SDK Development room:

It seems that in this case the server sends duplicates of the state events that are also in timeline in the state of the response of /v3/sync.

However this is still relevant in 2 other cases:

1. In /v3/sync, if the state events are received in state_after,
2. In simplified sliding sync, for the state events received in required_state.

In those cases it is normal to receive events that are already in timeline, since they should include all state changes since the last sync. However they shouldn't be received by event handlers twice.

I believe that we should only send state events in state_after or required_state, and ignore those in timeline, for state event handlers.