Another update from ingydotnet^djgoku

Task/Remove-duplicate-elements/Haskell/remove-duplicate-elements-3.hs

@@ -0,0 +1,8 @@
+import Data.Set
+
+unique :: Ord a => [a] -> [a]
+unique = loop empty
+  where
+    loop s []                    = []
+    loop s (x : xs) | member x s = loop s xs
+                    | otherwise  = x : loop (insert x s) xs