Double registrations…fact is, for such to be meaningful, to result in any change in any but local elections, so many people would have to do it, and do it on one side, that it would take some form of organized planning. The secret would never be safe and would, you can depend on it, be revealed somehow.
All it takes to police such activity is for the election board to send out a mailing every couple of years with a mandatory response if you are still at that address…and that, of course, will result in members of both parties being purged from the lists because, you know, people don’t always answer that sort of mail, or even look at it carefully, if at all. Well, perhaps slightly more Dems would get purged than Reps, though I’m not at all sure about that.
I suspect one block of voters who might be voting fraudulently are ex-felons, depending on the state (I don’t think the states have to abide by some Federal guideline on that, I could be wrong).
I doubt, though, that felons, in general, are so interested in performing a civic duty. Why? Get caught and go back to the joint? Besides, going to the election board to register and hanging out with the typical political appointee there might well be in violation of their parole particulars about association with known criminals…
Yes, I could see situations, particularly where automatic registration links with the DMV, or whatever, where undocumented folks could get registered and could be inspired to vote, figuring that if the county was doing their job, they wouldn’t have registered them in the first place. Since most of such folks are permanent, if not especially legal, residents, I don’t view voting by them as being the worst thing that ever happened.