It is so easy to commit voter registration fraud that it's would be a joke if it wasn't so sad. Voter fraud is a little more difficult, but because the penalities aren't too bad people do that, too. Most of the fraud is committed by Democrats, I have seen it. Unfortunately, it's harder to prove that there was a conspiracy rather than "an honest mistake". In one case down in southern California in the 1994 election cycle, I identified over 1,000 individuals -- 98% of them Democrats -- who were registered to vote more than once. The clincher was that all of them -- both the first and second and sometimes third -- registrations were within a few months of each other. Many of the people who actually registered two, three and more times were paid by the California Democrats to register voters.
Two-thirds of these people were taken off the voter rolls after the press did a big story on it and documentation was turned over to the Registrar. Sadly, not everyone was removed. if you are registered twice, you can vote twice, it's as simple as that.