If the user navigates away from the current page, bascially you should have session timeout mechanisim
Another action reposts the same page, it would be very rare chance to happen. Are you using Seam, if yes Seam will help you automatically take care of mutliple sessions established thru mulitple browsers by the same user.
Your code seems to be good, as long as you have a test case to test your functionality.
At least in RF 3.3.3 in FireFox if you click away from a page during a Richfaces Ajax query (i.e. its waiting for the server when you click away) you get a 599 error. It may have been different when I was using RF 3.1