It's just timing. More people noticed the second one. Maybe a slight recognition penalty by abbreviating HIBP, but even without that, the same thing has happened to me more than once.

Edit to add: There is a dupe detector that prevents duplicate submissions, but I'm not sure what the parameters are (e.g., detection window duration, whether it does title matching). Sometimes when I've submitted dupes, it returns the previous submission rather than creating a new one. Clearly that didn't kick in in this case.