Why does Firebase Cloud Messaging send after some interval?

Question

Google

Salute to all!

For some reason, Firebase Cloud Messaging started sending notifications not instantly, sometimes after an hour it comes.
Whoever faced this?
Another 1 cant started sending 2 notifications at a time:
First: normal
Second empty, with the following content:
Background Message Title
Background Message Body

Answer 1

This is why notifications are duplicated

messaging.onMessage(function (payload) {
            console.log(payload);
            const notificationOption={
                body:payload.notification.body,
                icon:payload.notification.icon
            };
            if(Notification.permission === "granted"){ 
                var notification = new Notification(payload.notification.title,notificationOption);
                notification.onclick=function (ev) {
                    ev.preventDefault();
                    window.open(payload.notification.click_action,'_blank');
                    notification.close();
                }
            }
        });

But without this piece, sending does not work at all
And the issue of late notifications from firebase has not been resolved