Placing a session limit on scripts

Can this be done? What I'd like to do is set a limit of once-per-script on my site for bigger/heavier pages. For example, I love MPDev's VBISpy, and I often see users with several tabs open to the page. I'd like them to only have ONE instance of the page/script, and any additional connections from that user would simply throw a "you are allowed only one instance of (script)" error.