code: func (server *Server) HasClientSubscribed(arg *SubscribeArg) bool {
  	if topicSubscribers, ok := server.subscribers[arg.Topic]; ok {
  		for _, topicSubscriber := range topicSubscribers {
  			if *topicSubscriber == *arg {
  				return true
  			}
  		}
  	}
  	return false
  }
docs: // HasClientSubscribed - True if a client subscribed to this server with the same topic