What I do in this case is that I store the parameters of the form, and if I find that I have already received it, I check compare the 2 sets of parameters. If they are identical then no problem, I do not process the second form and just resend the result. If there is a difference I send an error message. What you do in this case really depends on your application. You might want to "undo" the first transaction and process the second one.
Update: I forgot to mention that I store the unique identifier in a hidden parameter. This is certainly not completely secure, as the parameter can be changed and intercepted, but I believe it is OK in my case (malicious users changing the parameter would just loose the session and outside attackers trying to hijack the session would get the error message).
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