The truth: Double voting is extremely rare – which figures, since it can land you a 5-year prison sentence, a steep price to pay for one incremental vote. Accusations of double voting usually arise when voter lists are matched from one place to another: John Doe voted in County A; and look, John Doe voted in County B – double voting! Almost always, you're dealing with errors -- two different voters with the same name and birthdate, or people being counted as having voted when they haven't.
Typical case: In Missouri in 2000 and 2002, hundreds of voters were alleged to have voted twice either within the state or once in Kansas and once in Missouri. Reporters followed up on 18 Kansas City; 13 were shown to result from clerical errors. In total, public sources backed up only four cases, for a documented fraud rate of 0.0003 percent.