This was one of the easiest ones that I did. I knew from the beginning I new that a reduce and String.graphemes was the way to go.

Once you have the string I implemented this.
    def remove_duplicates(string) do
        string
        |> String.graphemes()
        |> Enum.reduce([], fn character, list -> 
            if character in list, do: list,
            else: [character | list]
        end)
        |> Enum.reverse()
    end

Take the string and turn it into a list, reduce over the list and prepend the character to the list if not in the accumulator, reverse the list.

That is about it... really.