Well, all of those solutions are fine, but this one takes the strain off the server's side. It uses JavaScript, and should be relatively easy to follow.
Here is the JavaScript portion:

var flag = 0;
function StopSubmit() {
  if(!flag) { flag++; return 1; } 
  return 0; 
}
[download]

And here is the form portion (you need to add the method and action parts):

<form onSubmit="return StopSubmit();">
[download]

The 15 year old, freshman programmer,
Stephen Rawls