OK, I had never heard of duck sauce before, but I assumed that duck sauce could be hoisin sauce because that is what they serve on Peking duck with the pancakes. According to Wiki, plum sauce (made from plums and kind of translucent orange) was mistakenly served with Peking Duck in America, earning the sobriquet duck sauce.
In my own case, it was exactly the opposite. When I first had Moo Shoo Pork, the people I was with called the condiment plum sauce, when it was in fact hoisin sauce. It was several years until I figured out the truth. By the way, hoisin sauce is actually made from soy beans.