Why are there two notifications in the service worker?

Question

Google

Good day, I don’t understand a little why two notifications come from the service worker,
the worker code itself:

importScripts('https://www.gstatic.com/firebasejs/3.5.2/firebase-app.js');
importScripts('https://www.gstatic.com/firebasejs/3.5.2/firebase-messaging.js');

firebase.initializeApp({
    'messagingSenderId': 'sender id'
});

var messaging = firebase.messaging();

self.addEventListener('push', function(event) {
    var json_data = JSON.parse(event.data.text());
    console.log('[Service Worker] Push JSON:', json_data);
    var not_title = json_data.notification.title;
    var not_data = {
        body: json_data.notification.body.length > 100 ? json_data.notification.body.substr(0, 97) + '...' : json_data.notification.body,
        icon: 'path to icon',
        vibrate: [200, 100, 200, 100, 200, 100, 200],
        click_action: json_data.notification.click_action
    };

    if(json_data.data) {
        // not_data.badge = payload.notification.data.image ? payload.notification.data.image : '';
        not_data.image = json_data.data.image ? json_data.data.image : '';
    }

   event.waitUntil(self.registration.showNotification(not_title, not_data));

});

in particular, I get both a custom notification and a regular one (which does not have an icon and an image)