input: Write a function to remove duplicates from a list while maintaining the order of the elements. If the given list is empty, return an empty list.
  assert remove_duplicates([1, 2, 3, 4, 3, 2, 1]) == [1, 2, 3, 4]
  assert remove_duplicates([4, 3, 1, 2, 3, 2, 1]) == [4, 3, 1, 2]
  assert remove_duplicates([1, 1, 1, 1]) == [1]
  assert remove_duplicates([]) == []
  assert remove_duplicates([5]) == [5]
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