input: Write a python function to extract duplicate numbers from a list.
  assert extractDuplicates([1, 2, 3, 2, 4, 5, 6, 1]) == [2, 1]
  assert extractDuplicates([10, 20, 20, 30, 40, 40, 50]) == [20, 40]
  assert extractDuplicates([5, 7, 7, 8, 5, 3, 3]) == [7, 5, 3]
output: def extractDuplicates(numbers):
      seen = set()
      duplicates = set()
      for number in numbers:
          if number in seen:
              duplicates.add(number)
          else:
              seen.add(number)
      return list(duplicates)