code: protected function validateDuplicates() {
      return (
        !$this->duplicatesExistForColumn($this->node->getQualifiedLeftColumnName()) &&
        !$this->duplicatesExistForColumn($this->node->getQualifiedRightColumnName())
      );
    }
docs: Checks that there are no duplicates for the `lft` and `rgt` columns.
  
  @return boolean