input: Write a function to merge two lists without duplicates.
  assert merge_lists([1, 2, 3], [2, 3, 4]) == [1, 2, 3, 4]
  assert merge_lists([5, 6, 7], [7, 8, 9]) == [5, 6, 7, 8, 9]
  assert merge_lists([1, 1, 1], [2, 2, 2]) == [1, 2]
output: def merge_lists(list1, list2):
      return list(set(list1 + list2))