It's possible that a non-rev could list themselves twice, though its not supposed to be done and they would never be given two boarding passes even if they did. Usually what happens is you get someone who doesn't know how things work so they list themselves for multiple flights on the same route, not realizing that if they miss the first flight that they'll be automatically rolled over to the next flight anyway. So then when the agent rolls the list over, they show up twice. But again, once it comes to actually clearing the seats the agent will just cancel one of the reservations anyway.
As someone mentioned it'd be quite pointless since non-revs only get the left over scraps 15 min before departure anyway. If they get called up to the counter for their seat assignment they can simply ask for the seat that has an empty one next to it.