Problem with posting

I'm writing a voting script, but I have a problem with it. Voters cast their vote by submitting a form. When this form is received it is added to the database, and a session variable is set saying that they've voted. The problem is that once someone's voted they can just press reload on their browser and it sends another vote. This happens in Netscape 4.x. All other browsers either do not submit form data or recognise that the session variable is set. Any ideas on how to fix this? Thanks. --jaa