Lancelot Hogben wrote a series of books, in the 60's I think, in which he said that dukes comes from the Romany "dukker, dukkerje" for hand, palmistry (My Romany is probably incorrect, but that's close). Our word "pal" is said to come from the Romany "chal" for cousin.
Hogben may be a funny name, but his Oxford bona fides were quite impressive, as were the books.