Well, it would be pretty difficult to duplicate the tiny presidents head thing that you can't see unless you hold it to a light...and the 'United States of America' type inside the collar if the head...
Plus the special classified paper they're made out of, no one has ever duplicted...Yes, people will always try to counterfeit money but most of the time they can only get away with it because some clerk isn't paying any attention to the bills handed to them...