uniquePerms :: (Eq a) => [a] -> [[a]]
uniquePerms = permBag . makeBag
makeBag :: (Eq a) => [a] -> Bag a
makeBag (a:as) = mix a $ makeBag as
mix a (bn@(b,n):bs) | a == b = (b,n+1):bs
| otherwise = bn : mix a bs
permBag :: Bag a -> [[a]]
permBag bs = concatMap (\(f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs
oneOfEach (an@(a,n):bs) =
let bs' = if n == 1 then bs else (a,n-1):bs
in (a,bs') : mapSnd (an:) (oneOfEach bs)