swa66 - 7:55 am on Feb 28, 2013 (gmt 0)
Track the session and the form itself.
Session tracking is obvious I guess.
To track the form: add a hidden field with a random value in it.
(the same you do for anti-CSRF)
When they hit back, the form will send back the same hidden randomvalue that was used in the earlier "reservation": you can safely ignore it as it's twice the same browser (nobody else will have the same random value).