Even if there were laws covering duplicate people, I think first you'd have to prove which one was the duplicate.
Just because one Riker made it back to the ship doesn't automatically make him the original. I haven't seen the episode in a while, but I don't recall that it makes that point explicit. The only reason they treated Will as the original is that he had a history with Starfleet during that time, and Tom didn't.
Unless there's some other way of proving it, then I'd say, yes, Tom had as much legal right to claim the persona as Will did. At least until someone wrote a new law to cover it.
there a way of proving that someone's not the original? If there is, then Harry Kim would be in for a hard time when Voyager got back.