In scp/tickets.php ~ line 78 AFTER
if(!$errors && ($respId=$ticket->postResponse($_POST,$_POST,$_POST,$_FILES,$cc,$bcc))){
$msg='Response Posted Successfully';
//Set status if any.
ADD
$assign_message = "Ticket assigned to ".$thisuser->getUsername();
if(($thisuser->isadmin() || $thisuser->isManager() || $thisuser->isStaff()) && $thisuser->getId()!=$ticket->getStaffId())
$ticket->assignStaff($thisuser->getId(),$assign_message);
I did some limited testing. It should assign it even if already assigned.