Generally the derailleur is fastened to the hanger solely with the bolt. I have seen the extra nut that you are wondering about on a few odd frames, mostly lower end stuff. In defense of the seller, every derailleur I've bought or sold had only the single bolt so I wouldn't slam them. Unless a superfluous nut is one of those weird French things that I am ignorant of, my advice is to put a drop of thread lock on the fixing bolt, mount it up, and ride that thing!