server.go
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
}
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
}
// HasClientSubscribed - True if a client subscribed to this server with the same topic