I question the "voting twice" claim. Ballots were supposed to be controled precint captians. To get a ballot, you had to ID yourself, and your name had to appear on the roll. Your name is checked off. I assume some people deposited more than one ballot, but that does not mean they voted more than one. For example, out of a man-woman couple, the man may have ofered to carry his better-half's ballot to a ballot box.
I wouldn't go so far as to claim nobody voted more than once, but doing so requires either a breakdown in ballot allocation, or one person converting another person's ballot for his own use. Control over the later is not as tight as in an election.