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

You could assign a 'mini-session' ID to each instance of the input form, then use AJAX to ping the server with that ID. If the user tries to request the same form when there's an active ID, it should display an error message. If the server doesn't hear the ping for a certain amount of time, expire the mini-session. (This is basically a very simple locking strategy)

As others has mentioned, you can't prevent the user from starting a new session without resorting to ActiveX or other nastiness. The basic problem is that there is no way for you to know whether a user closed the old browser window or left it open.

What you can do however, is to invalidate the previous session as soon as the user logs into a new (A bit similar to how may Instant Messaging clients behave).

On each login, assign a new GUID to the user in your database. Also store this GUID in the session cache (No need to ship it back and forth to the pages, which won't work for GET requests anyway). On each page request, compare the GUID assigned to the user in the database with the GUID in the session cache. If they don't match, return a "You have logged in from somewhere else" response.

Update I was a bit too fast on the trigger. This doesn't prevent the scenario where the user opens multiple tabs/windows within the same browser process. So you would have to combine this solution with Dave Anderson suggestion for storing a ViewState hash (or simply a GUID) so that only the last served page in a session is allowed to post back.

Security Update Also, you can only rely on this framework as a convenience to the user since it's not secure. Any half decent hacker will be able to circumvent these measures.

If someone copy the website url and paste it in a new window or tab, the browser history for that window/tab will be empty... so you can use javascript and check the history..

if (history.length == 1) {  //0 for IE, 1 for Firefox
    // This is a new window or a new tab.
}

Now you can prompt the user to close the tab, or disable that page (by making all elements disabled for example).