I think you'll find this pretty tricky. Because of the stateless nature of HTML based applications, you're going to run into a problem:
Let's say you have some mechanism to check for a second browser instance. Perhaps you use a cookie. Now you open a second browser and you get the page that tells you "you can't run this twice". So you close the browser and click on a link back in the first browser. Now you get the same "you can't run this twice" page. Why is this? It's because the server can not distinguish between different requests. It can not know that request number two is coming from one browser or another. Such is the environment of stateless applications. You'll need to write some kind of applet that will run at the browser that employs more complicated techniques.
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Peter
Application Multiple Instance
