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 of the same web app and click on buttons to read information that they have used before to enter into an input screen that they're currently working on.

What happens though is that it seems to screw up Session variables and then the user will update their previous work with their new work. Or they will delete their previous work all together or who knows...

EDIT I have seen this done before with online banking web applications. If you are already logged in, the new window will kindly tell you that you already have the app open. In my case, the user does not need to log in.

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?

Thanks

Answer 1

 <script type="text/javascript">
        window.onload = function(){
        if (document.cookie.indexOf("_instance=true") === -1) {
        document.cookie = "_instance=true";
        // Set the onunload function
        window.onunload = function(){
            document.cookie ="_instance=true;expires=Thu, 01-Jan-1970 00:00:01 GMT";
        };
        // Load the application
        }
        else {
             alert(" Security Alerts.You Are Opening Multiple Window. This window will now close.");
             var win = window.open("about:blank", "_self"); win.close();
        // Notify the user
        }
        };
    </script>