Hi there Marked,
<form method="POST" action="">
<input type="submit" value="Submit" name="submit"/>
if(isset($_POST['submit']) && ($_POST['submit'] == "Submit"))
execute some code when the form is submitted
This is what I use, and I haven't had a 'double submit' yet. Ie check the value of the key, then process.
Changing $_POST to $_GET is only good if you create the vars and pass them in the URL, $_POST is from the form submitted data.
I suppose there is some fancy js out there that would disable the submit button when it is actioned, but I have never tried this method.
Hope this helps..