input: Write a python function to find duplicates in a list.
  assert find_duplicates([1, 2, 3, 4, 5, 3, 2, 1]) == [3, 2, 1]
  assert find_duplicates([10, 20, 30, 40, 50, 40, 30, 20]) == [40, 30, 20]
  assert find_duplicates([7, 8, 9, 7, 8, 9]) == [7, 8, 9]
output: def find_duplicates(lst):
      seen = set()
      duplicates = set()
      for item in lst:
          if item in seen:
              duplicates.add(item)
          else:
              seen.add(item)
      return list(duplicates)