serialdispatcher.go
func (d *SerialDispatcher) AppendHandlers(handlers ...Handler) {
	d.mutex.Lock()
	defer d.mutex.Unlock()

Loop:
	for _, newHandler := range handlers {
		for _, existingHandler := range d.handlers {
			if newHandler == existingHandler {
				continue Loop
			}
		}
		d.handlers = append(d.handlers, newHandler)
	}
}
func (d *SerialDispatcher) AppendHandlers(handlers ...Handler) {
	d.mutex.Lock()
	defer d.mutex.Unlock()

Loop:
	for _, newHandler := range handlers {
		for _, existingHandler := range d.handlers {
			if newHandler == existingHandler {
				continue Loop
			}
		}
		d.handlers = append(d.handlers, newHandler)
	}
}
// AppendHandlers implements `Dispatcher.AppendHandlers`