multiple notificaition in gcm meteor

I have a simple query. I am implementing GCM notification in my application developed in meteor. The code goes as follows

    var gcm = Meteor.npmRequire('node-gcm');
    var sender = new gcm.Sender('MY_ID');
    var message = new gcm.Message();
    //message.addData('message',new Date().getTime());
    message.addData({'title':'mytitle','message':new Date().getTime()});
    message.delay_while_idle = true;
    var registrationIds = [];
    registrationIds.push("MY_REG_NO");
    sender.send(message, registrationIds, 4, function (err, result) {
        console.log(result);
    });

Now when I call this method from client, I get a notification properly. But when I invoke this method again, the previous notification is overridden with the latest one. How can I show a list of notifications in the notification tray without overriding the previous ones?

Thanks in advance.