input: Write a function to remove duplicates from a list. If the list is empty, return an empty list. The order of elements should be preserved.
  assert remove_duplicates([1, 2, 3, 4, 3, 2, 1])==[1, 2, 3, 4]
  assert remove_duplicates([5, 5, 5, 5])==[5]
  assert remove_duplicates([1])==[1]
  assert remove_duplicates([])==[]
  assert remove_duplicates([10, 20, 30, 40, 30, 20, 10])
  ==[10, 20, 30, 40]
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