Phonegap Pushnotification + node-gcm: group notifications

I have something like this in a Node app:

var sender = new gcm.Sender("XPTO");
var registrationIds = ["whatever"];
...
var message = new gcm.Message({
    data: {
        avatar: body_data.avatar,
        message: body_data.message
    }
});

sender.send(message, registrationIds, 4, function (err, result) { console.log("success"); });

It works fine, the notification arrives and goes to the tray if the app if not opened. But if I send a new notification to the same registrationId, the old notification is "updated" (or removed) and only the new one is shows.

If I add a random integer as parameter to notId

message.addData("notId", parseInt(Math.random() * 25));

the notifications are kept in the tray, but then the tray start to show multiple notifications. Is there a way to group the notifications?