How to prevent a user from having multiple instances of the Same Web application

Question

I\'m wondering if it is possible to determine if a user already has a web browser open to the web application I\'m working on. It seems that they can open several instances

Answer 1

Is there a simple way to determine if they already have a browser window open to the web application and if so, just close the browser or display a different page to let them know they can only have 1 open at a time?

In short, No.
Without writing some kind of activeX control, there is no code you could write that could stop the user from opening a (seperate instance of) IE/FF etc and having one instance detect the other.