It can be pretty slow as you are iterating through all elements twice. Instead it is better to use:
inc = & &1 + 1
Enum.reduce(enumerable, %{}, &Map.update(&2, &1, 1, inc))
But in this case if this is homework and you know that the constraints will always hold then XOR solution by the @mudasobwa. If you do not know whether it will integer-only or you do not know if there will be only one value that occurs odd number of times, then second solution with map (however I would use Map.has_key?/2 or Map.fetch/2 instead of Map.get/2 as it is less quirky).
