You can do a lot of things to prevent it. But you shouldn't have a player find that out the hard way ever, which is why I'm making this suggestion.
The fact that is HAS happened is an extremely bad experience for 2 people, and the fact that it's happened once means it will happen again. Even if it's drop in dropout it there should be guards preventing this accidentally happening.
a warning should be a bare minimum, I would say ideally you should keep the shuffle the last quicksave of a specific instance to a independent save whenever you save a new instance.
