This patch changes the behaviour of the `Timeline` when a duplicated
event is received. Prior to this patch, the old event was removed, and
the one was added. With this patch, the old event is kept, and the new
one is skipped.
This is important for https://github.com/matrix-org/matrix-rust-sdk/pull/3512
where events are mapped to the timeline item indices. When an event is
removed, it becomes difficult to keep the mapping correct.
This patch also adds duplication detection for pagination (with
`TimelineItemPosition::Start`).
Ivan Enderlin
15ed91e047