input: Write a function to remove duplicates from a list.
  assert remove_duplicates([5, 9, 2, 4, 5, 9, 2]) == [5, 9, 2, 4]
  assert remove_duplicates([1, 1, 1, 1]) == [1]
  assert remove_duplicates([2, 3, 4, 5, 6]) == [2, 3, 4, 5, 6]
  assert remove_duplicates([10, 20, 20, 30, 30, 40]) == [10, 20, 30, 40]
output: def remove_duplicates(lst):
      seen = set()
      result = []
      for item in lst:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result