code: func (s *StanServer) isDuplicateConnect(client *client) bool {
  	client.RLock()
  	hbInbox := client.info.HbInbox
  	client.RUnlock()
  
  	// This is the HbInbox from the "old" client. See if it is up and
  	// running by sending a ping to that inbox.
  	_, err := s.nc.Request(hbInbox, nil, s.dupCIDTimeout)
  
  	// If err is nil, the currently registered client responded, so this is a
  	// duplicate.
  	return err == nil
  }
docs: // isDuplicateConnect determines if the given client ID is a duplicate
  // connection by pinging the old client's heartbeat inbox and checking if it
  // responds. If it does, it's a duplicate connection.