A new room created by a new user sometimes contains duplicated timeline items

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* Existing user on Element X Android
* In Element Web:
* register a new user
* create a DM with the existing user
* send 2 messages
* In Element X Android, accept the invitation. The timeline will be displayed.
* Sometimes, there are duplicate items in the timeline:


Note: this only happens sometimes. I estimate about 50% of the time.

(The same thing happens if you use Element X iOS instead of Element X Android.)


I investigated this a bit and found that the duplicated items are provided via the `initial_values` from `TimelineSubscriber`, so they must be part of `state.items` within `matrix_sdk_ui::timeline::controller::TimelineController::subscribe`. I don't know why they appear, but I suspect we are updating the timeline in 2 different ways simultaneously, and some kind of race is happening.

(Found while testing https://github.com/element-hq/crypto-internal/issues/398 .)

Reproduce the issue with a new Element Web user and an existing Element X Android or iOS user, then inspect matrix_sdk_ui::timeline::controller::TimelineController::subscribe and the TimelineSubscriber initial_values path. Trace how state.items is populated during room creation and accepting the invitation; done means the initial timeline contains no duplicated items.

Written by the indexing model from the issue text.