input: Write a function to combine two lists and remove duplicates.
  assert combine_lists([1, 2, 3], [2, 3, 4]) == [1, 2, 3, 4]
  assert combine_lists([5, 5, 6], [6, 7, 8]) == [5, 6, 7, 8]
  assert combine_lists([10, 20, 30], [30, 40, 50]) == [10, 20, 30, 40, 50]
output: def combine_lists(list1, list2):
      combined_list = list1 + list2
      return list(dict.fromkeys(combined_list))