In the arcade version of Donkey Kong Jr., the opening depicts Mario and a lookalike lifting Donkey Kong (who would later become Cranky Kong) into a cage. The most logical explanation for this is that the lookalike is, in fact, Luigi, marking this as his first, albeit unnamed, appearance. The lookalike makes no further appearance throughout the remainder of the game.
I found this quote in the Mario Wiki. All I can say is, Pics or it didn't happen.