In the Haskell version of the problem, you use both Int and Integer:

countBy :: Integer -> Int -> [Integer]

Why mix Int and Integer in the types? I think they should both be the same.

Here's a test case that uses random integer lists. (I forget the syntax to make it work for any list of Eq.)

module RemoveDuplicates.Test where
import Data.List (nub)
import RemoveDuplicates (unique)
import Test.Hspec

main = hspec $ do
  describe "Duplicates removed from array" $ do
    it "should work for random arrays" $
      property $ \a -> unique a == (nub a :: [Int])

It'd help a lot if you could explain that <> is a new synonym for mappend (I don't think it was originally the case, right?)

I searched for <> with Hoogle, and it couldn't find this synonym:

http://www.haskell.org/hoogle/?hoogle=%3C%3E

Why does this say the Haskell binary monoid operator is <>? I thought it was mappend?

I would love to build on this to make a Unicode-safe version of the question!

Example Unicode-safe solution in Haskell at http://www.codewars.com/kata/reviews/54179e681d3bf3e16b000009/groups/542f4bbcd002f889540008a6 .