How to group the fcm push notifications like whats app

I am using firebase push notifications to send the notifications from web to mobile. I refer from this Documentaion to create the notification. If the notification trigger once it create the multiple notification from same chat.

But i need to group the all notification based on chatid. my push notification data.

var fcmmessage = {
    to: device_token,
    notification: {
      title: sender,
      body: message,
      sound: "default",
      click_action: "FCM_PLUGIN_ACTIVITY",
      tag:  chatid
    }
  };

I used the tag property to group the notification.But it rewrite the before notification. I want to group the all notification based on chatid.