Okay so it's sorta pointless considering that even if there was support for this you could still open more than one instance if you have more than one browser installed, but I was wondering anyway.
If it is a webapp they need to log into I would just restrict a user from being logged in more than once at a time. Other than that would wouldn't want to restrict their opening of the actual window
I hope not. This restriction would be a pain to users and only underline a problem in the Web Application Infrastructure.
What are you trying to achieve by restricting them?