code: def _validate_duplication(self, subj_and_pred, cl):
          
          subj, pred = subj_and_pred
  
          log.info("Validating duplication of member %s" % pred)
          if (subj, pred) in self.checked_attributes:
              err = self.err("{0} - duplicated member of {1}",
                             self._field_name_from_uri(pred),
                             self._field_name_from_uri(cl))
              return ValidationWarning(ValidationResult.WARNING, err['err'],
                                       err['line'], err['num'])
docs: returns error if we've already seen the member `pred` on `subj`