Fix deduplicating of membership events to not create unused state groups. (#17164)

We try and deduplicate in two places: 1) really early on, and 2) just
before we persist the event. The first case was broken due to it
occuring before the profile information was added, and so it thought the
event contents were different.

The second case did catch it and handle it correctly, however doing so
creates a redundant state group leading to bloat.

Fixes #3791