code: def duplicate_items(*collections):
      
      duplicates = set()
      seen = set()
      for item in flatten(collections):
          if item in seen:
              duplicates.add(item)
          else:
              seen.add(item)
      return duplicates
docs: Search for duplicate items in all collections.
  
      Examples
      --------
      >>> duplicate_items([1, 2], [3])
      set()
      >>> duplicate_items({1: 'a', 2: 'a'})
      set()
      >>> duplicate_items(['a', 'b', 'a'])
      {'a'}
      >>> duplicate_items([1, 2], {3: 'hi', 4: 'ha'}, (2, 3))
      {2, 3}