Without any means of cross checking, I would think this is very easy to do. You have people who have:
Moved to a different county and are still registered in their old county.
Moved to a different State and are still registered in their old State
Have homes in two States.
Live overseas but are still registered in a State.
Are college student registered at their parents address and newly register at their college.
Have recently deceased relatives who are still registered to vote (in this case they may even still have the deceased’s drivers license or other form of ID).
It’s an interesting question. A lot of retirees have two homes or residences: one in the north, and the other in the south e.g., Arizona, where they spend their winters. Obviously, you can only vote for president once, but you could make an argument for something like “dual citizenship” in two states where you spend at least four months of the year, in which you would be allowed to vote in local (maybe even state) elections in both places. It might not be a good idea, but I don’t think it’s a crazy idea. Even in our current system, national elections inevitably give people a voice as to what goes on in other states, for better or worse.