input: Write a python function to remove duplicates from a list while maintaining the order of elements.
  assert remove_duplicates([1, 1, 3, 4, 5, 6, 6, 7, 8, 9, 9]) == [1, 3, 4, 5, 6, 7, 8, 9]
  assert remove_duplicates([2, 2, 2, 3, 4, 4, 5]) == [2, 3, 4, 5]
  assert remove_duplicates([10, 10, 9, 8, 8, 7, 6]) == [10, 9, 8, 7, 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