Using a session, you set a value indicating that a particular form is going to be submitted, then when submitted you clear or remove the value.
The easier solution is to change you're get action to post action, this will prompt the user. They can still submit again, but you can still use sessions or access token techniques to handle those.
$_SESSION['frmPostReply'] = true;
<form method="get" action="postReply.php">
<!-- Some fields -->
if (!isset($_SESSION['frmPostReply']) || !$_SESSION['frmPostReply'])
print 'Please use the form.';
// Or you can header('Location: http://yoursite.com/yourform.php');
$_SESSION['frmPostReply'] = false;
// Do you're stuff.
This article has a simple access token technique: http://www.phpro.org/tutorials/Preve...e-Submits.html