That's surprisingly complicated: since different browsers do not share cookies (i.e. Chrome cookies are not used by IE and vice versa) it has to be done at the server, and since different browsers will also have different sessions, it has to be pretty much DB related.
The only way I can think of off the top of my head is to store the client IP and session ID in your DB when he logs in, and check for live session using the same IP when he tries again.
There is code here that will retrieve all active sessions:
http://stackoverflow.com/questions/1470334/list-all-active-asp-net-sessions[
^], you can then check a cookie to see if this is the same browser or not.
Submit an article or tip