If you don't have enough information check with your local party to see if they can find out if he is registered and where. In addition, they should be able to let you know if he votes at that address.
More than likely, there shouldn't be any double registration as the voter law required states to have a state voter registration system.
** Government is not the problem. Republicans are the problem. ** "Christianity neither is, nor ever was a part of the common law." --Thomas Jefferson, Feb 10, 1814