If sending 1000 notifications or even more, then NotificationSender It will be created 1000 times, $this->events->listen(NotificationFailed::class, fn () => $this->failedEventWasDispatched = true); It will also be executed 1000 times. events bigger and bigger.
If fired NotificationFailed::class Will execute anonymous function...many, many,many times

it's no go, it may not align with the author's ideas. the author should be trying to solve the new NotificationSender fired NotificationFailed once, But there are many new NotificationSender

@ranson-xu
static::$registeredNotificationFailedListener
it is static so it does not depend on how many new NotificationSender are executed.

    protected static bool $registeredNotificationFailedListener = false;

        if (! $this->failedEventWasDispatched) {
            if ($exception instanceof HttpTransportException) {
                $exception = new TransportException($exception->getMessage(), $exception->getCode());
            }

            $this->events->dispatch(
                new NotificationFailed($notifiable, $notification, $channel, ['exception' => $exception])
            );
        }

NotificationSender line 165 to 173.
$this->events->listen(NotificationFailed::class, fn () => $this->failedEventWasDispatched = true); for update $this->failedEventWasDispatched. all NotificationSender registered only once, other NotificationSender how to do? or what is the purpose of this if.

the premise is "Queueing Notifications"

@ranson-xu PR #55507 (which introduced that if clause) has some history about it.

The gist is, Laravel never dispatched the NotificationFailed event by itself, but some 3rd-party/community channel drivers did.

So in addition to starting to dispatch the NotificationFailed event, we had to guard against double dispatching (once by the 3rd-party drivers, the other by the framework) to avoid requiring those packages to be updated on a patch release.

I understand. i suspect an increasestatic::$registeredNotificationFailedListener It cannot solve the problem.

#58452 goooood