Form refresh problem

Hi Guys I'm having a bit of a problem with form submission. I'm creating an opinion poll, and the user's vote is submitted as a form back to the originating page. I'm using !isset to see if the form has been submitted and if so, updating the database and showing the current results My problem is that a user, having voted once, can keel clicking refresh in their browser and the form data keeps getting sent, incrementing the poll results each time Is there a way to kill form data once it;s been posted, or is there a better way of doing what I am trying to accomplish? Many thanks Lucas Young