Depends how it's done. In France, we register only once in the country, in our "voting neighborhood".

Once you enter the voting building, you give your ID, sign the register, go to the voting cell, put your vote in an envelope, give your sealed envelope to be counted in a sealed box that has a mechanical counter for each envelope it gets.

Unhackable, because anyone can check the counter, count the signatures, and participate to the closing count of cast votes.

The real issue is if you can get inscribed on two different voting neighborhoods though... is that what you describe? (which shouldn't happen, given how long it takes to get your name on one voting list, since the gvt probably crosses your name out of all other voting neighborhoods when you do)