EndJoinGroup event is triggered even if joining failed

The activity plugin uses the EndJoinGroup event to create a notice automatically if someone joins a group.

With regularity I join a group by mistake I'm already a member of. The notice however is still created because the event is still triggered.
There are two alternatives:
1. Trigger the event, but allow some inspection of the result;
2. Don't trigger the event

The latter has my preference, because there really was no joining involved here.