input: Write a python function to identify all the duplicate elements in the given list.
  assert find_duplicates([1, 2, 3, 2, 1]) == [1, 2]
  assert find_duplicates([4, 5, 6, 7, 8, 8, 9, 9]) == [8, 9]
  assert find_duplicates([10, 11, 12, 13, 14, 15]) == []
  assert find_duplicates([20, 21, 21, 22, 23, 23, 24]) == [21, 23]
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)