Event handlers for state events are sometimes called twice

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.

Start by reading Client::handle_sync_timeline_events and Client::handle_sync_state_events, then trace how a state event reaches each path during sync. Confirm the duplicate-callback case with an OriginalRoomMemberEvent and define done as each state event handler being called once per arrival.

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