input: Write a function to remove duplicates from a list while preserving order.
  assert remove_duplicates([10, 20, 30, 10, 40, 20]) == [10, 20, 30, 40]
  assert remove_duplicates([1, 2, 2, 3, 4, 4, 5]) == [1, 2, 3, 4, 5]
  assert remove_duplicates([7, 7, 7, 8, 8, 9]) == [7, 8, 9]
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