input: Write a function to remove duplicate elements from a list.
  assert remove_duplicates([1, 2, 2, 3, 4, 4, 5]) == [1, 2, 3, 4, 5]
  assert remove_duplicates([10, 20, 20, 30, 40, 40, 50]) == [10, 20, 30, 40, 50]
  assert remove_duplicates([1, 1, 1, 1, 1]) == [1]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))