input: Write a python function to find duplicates in a list.
  assert find_duplicates([1, 2, 3, 3, 4, 5, 5, 6]) == [3, 5]
  assert find_duplicates([10, 20, 30, 20, 10, 40, 50]) == [20, 10]
  assert find_duplicates([5, 6, 7, 8, 9]) == []
output: def find_duplicates(lst):
      duplicates = []
      seen = set()
      for item in lst:
          if item in seen:
              if item not in duplicates:
                  duplicates.append(item)
          else:
              seen.add(item)
      return duplicates