Just disable the submit button using javascript once it is submitted.
use this code
put this is the head section of the page with the form.
<script language="JavaScript" type="text/JavaScript">
function disablebutton() {
document.form1.Submit.disabled = true;
return true;
}
</script>
form1 above is the name of your form. You should change this to whatever your form name is.
add this to the form tag of the form
onsubmit="return disablebutton();
//example
<form action="/go.php" method="post" enctype="multipart/form-data" name="form1" onsubmit="return disablebutton();">
should grey out the submit button so it can only be pressed once.