So I'm happy to add another bronze badge to my collection for this stupendously awesome question... but not quite this happy:
They don't all fit in the screenshot, but at time of writing I have 26 (twenty-six) identical notifications. (o_O;
Everything else seems to work fine, including a single notification for another "Popular Question" badge.
As @Jan mentioned, this was a systemwide bug in the whole SE network, and was mentioned on Meta SE today:
We had a problem with badges failing to award last night, so I added some extra logging to capture more info when it stumbled. In the process I accidentally broke something such that it appeared to work, but really didn't actually do anything. Deploying the fix now. Sorry folks.
It has since been fixed.
