input: Write a function to remove duplicates from a list while preserving the order.
  assert remove_duplicates([1, 3, 2, 1, 5, 4, 8, 6, 7, 5])==[1, 3, 2, 5, 4, 8, 6, 7]
  assert remove_duplicates([10, 20, 10, 30, 20, 40])==[10, 20, 30, 40]
  assert remove_duplicates([1, 1, 1, 1, 1, 1])==[1]
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