If auto reply message looping is allowed it has the potential to create bounces between GW and an external account. If the auto reply is set to send only a single reply, users are expected to recall that the person they have mailed has already told them they are away. After a few days most people seem to have forgotten this!
Somewhere GW must retain a list of addresses to which it has sent an autoreply. If this list can be deleted on a scheduled basis, auto replies can be sent more than once without the need to allow looping. Deleting this list daily/weekly would make some sort of sense.