What looks like a single event of applying a few labels is actually
multiple events, and the raciness is racy as we've recently experienced.
One solution would've been adding a “first responders notified” label
for enforcing consistency but issues already have enough labels (and we
could take it off by mistake), so a bot reaction will instead serve as a
marker for any later runs.

So if we're applying multiple labels or changing our mind about, say, an
area label after the notification has already been sent, this should
work fine now, without duplicate notifications.

Release Notes:

- N/A