The 144 character “tweet” format is creating some confusion. The point is not that anyone could determine HOW someone had voted, but that an individual appeared in person at a polling station and was told that a ballot had already been cast in that person’s name.
In other words, one possible way this happened is that someone sent in a fraudulent absentee voter application in, say, Fred Smith’s name, voted it, and mailed it. Then, when the real Fred Smith turns up at his precinct and tries to vote, the poll workers see that Fred Smith has already returned an absentee ballot.
Well, it’s a near surety that if a fraudulent vote was cast, it was cast for 0bama.