[Haskell-cafe] Permutations

OK, so here's something just for fun: Given a list of items, find all possible *unique* permutations of that list. (E.g., the input list is explicitly _allowed_ to contain duplicates. The output list should not contain any duplicate permutations.) I've found one simple way to do this, but I'm sure there are multiple valid approaches. So let's see what people come up with. ;-)