you could edit the code to use the time stamp for the transaction id instead of the session_id:
on the confirm page, change:
$AuthNet_required[0][$nextIndex] = "x_invoice_num";
$AuthNet_required[1][$nextIndex] = "".session_id() ."";
to:
$AuthNet_required[0][$nextIndex] = "x_invoice_num";
$AuthNet_required[1][$nextIndex] = "".time() ."";