Hours before President Trump declared he was launching a "major investigation into voter fraud, including those registered to vote in two states," the Herald-Tribune broke the news that Trump's senior White House adviser Steve Bannon is, in fact, registered to vote in both Florida and New York. Bannon did not vote in both states, which would be against the law; he only voted in New York.
However, his dual registration does appear to be in conflict with the U.S. government's stated rules on voting:
You can't be registered to vote in more than one place at a time. When you register to vote in a new location, you'll be asked for your previous address. Your new election office will send a cancellation form to your previous election office. [USA.gov]
The rule does seem to suggest the onus would fall on the Florida election office — and not on Bannon — to have cancelled his Florida voter registration after he registered in New York. And Bannon is far from the only American registered to vote in two states: A 2012 Pew Research Center study estimated that 2.75 million Americans are registered in "more than one state."
New York State's Board of Elections specifies on its official website under "qualifications to register to vote" that a person cannot "claim the right to vote elsewhere." This isn't the first time Bannon's voter registration has made the news: Last summer, it was reported that Bannon's registered address in Florida was a vacant home he never lived in, though The Guardian reported he's since changed his Florida address to that of a friend.