Use "string:x-canonical-private-synchronous:my-notification" hint to replace notificatios
It would be nice if notify-send would respect the non-standard hint "x-canonical-private-synchronous" for syncing notifications. Many other notification daemons already do that. It also solves the problem that libnotify apparently doesn't allow you to set a replace_id. Other notification daemons also accept the notification id as replace_id, but somehow this doesn't work with xfce4-notifyd.
Let me know, what you think of it, Regards