I'm confused. The way I understand it is that once the double checkmark appears, those messages are locked in and never rekeyed. That means once you see those checkmarks, you're guaranteed no one can snoop on that message anymore.

Assuming you have the notification on (Which anyone who cares about security could and should turn on), once you see a warning, you could just delete all messages that don't have the double checkmark (if any) and not send new ones.