Okay, I'll try to keep this in mind. So I just ask them if they had a villager move out recently or in boxes? .
Yes. Ask if they've been to someone's town since their last move out (if they're coming to adopt, they must have had someone leave recently), and if they haven't, ask them to go visit anyone and clean their void before coming to your town. If they pop up at any other town and the game is saved, their void will be clear and they can come and go without leaving any surprise villagers. x)