If we’re talking about file and folder diff tools, not necessarily for keeping duplicates off your drive, I have to plug Araxis Merge. It is a very expensive app. But it is native, fast, supports 3-way comparisons, and is simply the best diff tool I have used on Mac.
In terms of diff, Kaleidoscope is pretty but simply not standout powerful. Beyond Compare is powerful and affordable but slow and lacks 3-way comparison. DeltaWalker, which I also have, is extra slow due to being Java. Unless you only need diff occasionally and get DeltaWalker on sale, or really need 3-way comparison without breaking the bank, I would actually advise against this one. Araxis Merge just hits everything out of the park. At a very steep price.