What would you call a UX entity for which only one instance can exist?

I'm working on a notification platform and found a somewhat amusing bug. My software checks for updates every N minutes, and if there is an update, lets the user know via a non-modal notification. The amusing part is that more than one notification can stack, and I didn't sanity check, left my software for an hour and came back to a handful of notifications all telling me i needed to update.

So I'm working up beefing up the notification center to allow for a single-instance notification. That said, I'm just honestly not sure what to call this new feature of the user interface. In programming Jargon I'd call it a singleton, but the definition doesn't exactly carry over.