That sounds like another race condition where an individual client queries the server for an instance at the same time. Because the server doesn't have one for the group yet it creates several per client request.

Seems like the phasing should have a sanity wait on it. Something of a few milliseconds to sleep, then wake and then check for queries. Either that or proactively flag characters which are grouped. So it knows to create one instance for the group leader regardless of who hits the area first.