We actually assumed that we are only going to be verifying a single day's worth of exists at any given point. But the scenario you've mentioned is actually interesting. 

If Stacktach is down for an entire day, would we not have started sending glance exists messages directly from Yagi to the downstream systems? In that case we would have to ensure that we don't send notifications for the earlier messages. Would we still run into this issue in that case? 

I'll fix the flow anyway to pick all the pending exists messages and then group them by owner and audit_period beginning and ending and send one notification for each such group.