How can I prevent this?

[Orgy]

You should be redirecting your users away from the form after it has been validated and executed. Doing so will mean that if your user decides to spam the refresh button they'll be doing so on a safe page. How it might look in pseudocode:

Code:

if form submitted then
  validate form

  if form validates successfully then
    redirect user

Hey, that would work pretty nicely. But that would mean that each submissApproachion would require its own success page, and if done half-assed (like most of my scripts) it can get messy real quick