Allow storing the same event in multiple linked chunks of the same room in `IndexeddbEventCacheStore`

[mgoldenberg]

Recently, this behavior was implemented for SQLiteEventCacheStore in #6065. This change, however, was not extended to IndexeddbEventCacheStore and my assumption is that it would be good to maintain behavioral parity between these stores. So, this behavior should be implemented in IndexeddbEventCacheStore as well.

That being said, this change is part of a larger issue (see #5954), so it may make sense to wait until that issue is closed in case other changes to SQLiteEventCacheStore are coming soon.