That's my line. Been complaining about dual registration for years. It is quite easy to do when you have two places of residence and they are not crossmatched at the voting booth.
There are newspaper articles about that very subject. I believe they were printed after the 2000 presidential election debacle. No big lib paper such as the NY Times picked up on these investigations, of course.
Some info:
http://www.gregpalast.com/detail.cfm?artid=314
Excerpt:
The New York Times has uncovered Katherine Harris' wipe out of thousands of voters ... only three years after killing the story exposed by the BBC TV and the Guardian. Here, 'News Dissector' Schechter, formerly of ABC's 20/20, asks how the Times could editorialize on a story they never ran in the first place. And note the Times still can't bring itself to say that the color of illegally purged voters is ... Black.
I recall reading that it was about 46,000 people....cetainly enough to affect a close election in FL.
I wonder how many "snowbirds" vote twice. It would be easy for them to register in both their summer home state and their winter home state.