Notification system with Firebase triggers on callback on every reload

So I'm trying to make a notification system similar to Facebook's system. I have a cron job running in the background and when it feels the need to send out a notification to an user it uses the PHP (REST) SDK to send a push to Firebase. On the dashboard I have an 'on' callback that fetches all incoming notifications for the user and shows a notification for each incoming new notification

var push_ref = firebase_ref.on('child_added', function(push) {
    new PNotify({
        title: 'Notification',
        text: push.val().message,
        ...
    });
});

However, when I reload the page all previously sent notifications pops up simultaneously and fills the screen with notification boxes. I don't wanna delete the sent push notifications because later on I'm gonna make a system to fetch them and put them in a dropdown box (similar to the notification dropdown on Facebook).

My question is;

How can I keep all sent notifications but at the same time disable that Firebase returns all notifications when the client goes online again?