code: func (ipt *IPTables) AppendUnique(table string, args ...string) error {
  	exists, err := ipt.Exists(table, args...)
  	if err != nil {
  		return err
  	}
  
  	if !exists {
  		return ipt.Append(table, args...)
  	}
  
  	return nil
  }
docs: // AppendUnique acts like Append except that it won't add a duplicate