Joining a MUC with a duplicate nickname results in strange behavior.
When a user joins a MUC, using a nickname that is already being used by someone else in that MUC, a bug is triggered.
The user that appears to join last is shown an error, indicating that there's a conflict. However, unseen by this user, the room is joined regardless (with a numeric appendage to the original nickname).
The user should either get an error and not join, or the user should join with a modified nickname.