new_contents: 
  from collections import Counter
  
  def checkio(data):
      count = Counter( data )
      return [ element for element in data if count[element] > 1 ]
  
  
  
  if __name__ == "__main__":
      #These "asserts" using only for self-checking and not necessary for auto-testing
      assert isinstance(checkio([1]), list), "The result must be a list"
      assert checkio([1, 2, 3, 1, 3]) == [1, 3, 1, 3], "1st example"
      assert checkio([1, 2, 3, 4, 5]) == [], "2nd example"
      assert checkio([5, 5, 5, 5, 5]) == [5, 5, 5, 5, 5], "3rd example"
      assert checkio([10, 9, 10, 10, 9, 8]) == [10, 9, 10, 10, 9], "4th example"
  
subject: Create non-unique-elements.py