input: Write a function to remove duplicate elements from a list while maintaining the original order.
  assert remove_duplicates([1, 20, 4, 6, 20, 4, 1, 6]) == [1, 20, 4, 6]
  assert remove_duplicates([5, 5, 5, 5]) == [5]
  assert remove_duplicates([3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]) == [3, 1, 4, 5, 9, 2, 6]
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