iwish.hs

dedup [] = []
dedup (x:xs) = dedup' x xs
  where
    dedup' x [] = [x]
    dedup' x (x:xs) = dedup' x xs
    dedup' y (x:xs) = y : dedup' x xs

reality.hs

dedup [] = []
dedup (x:xs) = dedup' x xs
  where
    dedup' x [] = [x]
    dedup' y (x:xs) | x == y = dedup' x xs
                    | otherwise = y : dedup' x xs