window.focus() creates new tab when there are two opened

I have a javascript application that shows notifications regularly. When a notification is clicked, I want it to focus on the tab/window that my application is currently running. I can get that behavior working fine with this code:

            var options = {
                body: msg.body,
                sticky: true,
                icon : 'img/logo_quad.png',
                tag: id,
                renotify : true
            };

            var notification = new Notification(msg.title,options);

            notification.onclick = function(){
                window.focus();
            };

However, if I have two tabs running this same application at once, when I click it, instead of focusing one of the two tabs it opens a new tab with the same URL.

Why is that happening and how can I prevent it?

Note: A chrome only solution is welcome, I don't care about other browsers