My experience has been the opposite. Sweet & sour sauce is the grotesquely sweet one that is spooned over the popular S&S dishes (not usually available as a table condiment), and duck sauce has a mildly sweet, fruity flavor and used almost exclusively as a condiment for dipping. Sweet & sour is usually very dark orange, sometimes red. Duck sauce is light orange and translucent. It’s made from plums, apricots or peaches (sometimes all three) and originally meant to be used ON duck. There is another type of duck sauce called anitra in sugo that is made from ducks, and meant to be used on pasta.