On rare occasions, the PHP server may continu processing the page after the header function is called to redirect to the failure page
this can be fixed by finding the following code on the confirm page:
else if ("checkout_failure.php" != "") {
header("Location: ". "checkout_failure.php");
}
and adding an exit function:
else if ("checkout_failure.php" != "") {
header("Location: ". "checkout_failure.php");
}