This is the same notification bug as the one reported by me concerning Android’s download manager.notifications.
The root cause is that after an Android notification is displayed, its status doesn’t change to “read” or something like that, it is still considered as “new” and is placed in a kind of queue. When there’s another Android notification, all notifications from this queue are shown one by one. It takes tapping a notification to dismiss it, or else it remains in such a queue and is re-displayed eternally.
This bug is not unique/specific to just Android messaging apps, it affects all kinds of Android notifications. In my case it affects Android’s download manager and causes that until I manually tap each notification about a successful download, all the past notifications are re-shown on every new file download.