code: def parse_unique_urlencoded(content):
      
      urlencoded_params = urllib.parse.parse_qs(content)
      params = {}
      for key, value in six.iteritems(urlencoded_params):
          if len(value) != 1:
              msg = ('URL-encoded content contains a repeated value:'
                     '%s -> %s' % (key, ', '.join(value)))
              raise ValueError(msg)
          params[key] = value[0]
      return params
docs: Parses unique key-value parameters from urlencoded content.
  
      Args:
          content: string, URL-encoded key-value pairs.
  
      Returns:
          dict, The key-value pairs from ``content``.
  
      Raises:
          ValueError: if one of the keys is repeated.