Does it stay on after you Mark it all as Read? Sometimes very old messages arrive and they are all the way back in the list, but they are still unseen for that person.
Discussion
Scrolling up and down helped, but sometimes the same message / notification appears after definitely checking it.
My dot fixed after I marked all read. I was having the back button issue yesterday, but I broke phone and had to get a new one and I haven't had the issue on the new one yet.
My old phone had multiple accounts, but I've only added the primary so far. I can add the second to see if it duplicates if that helps you out.