input: Write a function to find duplicate numbers in a list.
  assert find_duplicates([10, 20, 10, 30, 20, 40]) == [10, 20]
  assert find_duplicates([1, 2, 3, 4, 5, 5, 6, 6, 7, 8]) == [5, 6]
  assert find_duplicates([100, 200, 300, 400, 200, 500, 600, 300]) == [200, 300]
output: def find_duplicates(lst):
      seen = set()
      duplicates = set()
      for number in lst:
          if number in seen:
              duplicates.add(number)
          else:
              seen.add(number)
      return list(duplicates)