>not sure I understand your uniqueness condition, either
>user account?
>machine?
>browser brand?
>a combination of these above?
>
>you could:
>- have a hidden field in the page that you send with AJAX requests
>- add a query string parameter to all URLs internal to the site (encoded as a GUID to avoid identity thief)

It is for a unique identifier based on the browser. It is not to recognize the browser. As, many are using the same browser. It is to recognize a unique identifier per browser installation. That way, is someone logs in twice with two accounts, with the 2nd login, I will be able to kill the session on the server side which was created from the first login. Then, on the first login next hit, he will be redirected to the main page with a message.