input: 
  def find_common_elements(list1: List[int], list2: List[int]) -> List[int]:
      """
      Find common elements in two given lists.
      >>> find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])
      [21]
      """
  
output:     common_elements = []
      for element in list1:
          if element in list2 and element not in common_elements:
              common_elements.append(element)
      return common_elements