input: def print_unique(my_list):
      """Prints each unique element of a given list once.
      >>> print_unique([1, 2, 2, 3, 4, 4, 5])
      1
      2
      3
      4
      5
      >>> print_unique(['a', 'b', 'b', 'c', 'd', 'd'])
      a
      b
      c
      d
      """
output:     seen = set()
      for item in my_list:
          if item not in seen:
              print(item)
              seen.add(item)