Fix some react warnings in GroupMemberList

[lukebarnard1]

• If the list contains two users twice, react would warn about duplicate keys. Use index instead.
• Check if unmounted before setting state after fetching members.

[dbkr]

Hmm, using list index as a key is a surefire way to confuse React and React doesn't like you doing it (because it gives it no more information than it would have otherwise). Why would we ever have duplicate users in the list?

[lukebarnard1]

Hmm, using list index as a key is a surefire way to confuse React and React doesn't like you doing it (because it gives it no more information than it would have otherwise)

Ah. I had no idea about this but that makes a lot of sense.

Why would we ever have duplicate users in the list?

The API allows it (currently). E.g. Amandine is +matrix twice.

[dbkr]

It's presumably not meaningful for the same user to be in the list twice though? I would confer with Erik, but I think I would vote for ignoring the dupes until synapse gets fixed to not send dupes?

[lukebarnard1]

ignoring the dupes until synapse gets fixed to not send dupes?

This is fair. And then userId would be an OK key.

[dbkr]

ES6 has Set() ftr :)