What year? Duster/Dart Sport/Demon bumpers should interchange as long as they're the same year. I think 74-76 were the same, as well as 70-72. 73 I can't remember.
Since the rear of the Dusters and Demons are the same, I would think that you could use any year, although you would want to use the brackets to match the bumper. Not sure if you might have to change mounting location on body though.