More than one instance of our chat in browser. Security prob

[mortek]

We are talking a lot about performance, and I want to do a important step for this namely to block next instance of chat for one user.
Basicaly I got a auth system and usernames are frozen. No guest allowed. But - user can open few the same chats (and same channel) in browser windows it can crash a server if he do this ex. 100 times!

Do you got any ideas how to allow user to open only one instance od chat (if its open allready in some browser window we block chat load?)
I think its very important challenge to all of us..