input: Write a function to remove all duplicate elements from a list while preserving the original order of elements. If the list is empty, return an empty list.
  assert remove_duplicates([1, 3, 3, 5, 5, 7]) == [1, 3, 5, 7]
  assert remove_duplicates([4, 4, 4, 4]) == [4]
  assert remove_duplicates([3, 2, 3, 1, 2, 3]) == [3, 2, 1]
  assert remove_duplicates([]) == []
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