input: Write a function to remove duplicates from a list. If the input list is empty, return an empty list.
  assert remove_duplicates([1, 2, 3, 4, 3, 2, 1])==[1, 2, 3, 4]
  assert remove_duplicates([3, 5, 10, 1, 2, 10])==[3, 5, 10, 1, 2]
  assert remove_duplicates([4, 3, 1, 2, 2, 4])==[4, 3, 1, 2]
  assert remove_duplicates([])==[]
  assert remove_duplicates([1, 1, 1, 1])==[1]
output: def remove_duplicates(lst):
      if not lst:
          return []
      seen = set()
      result = []
      for item in lst:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result