My initial problem was that an alert that was still active was marked as cleared, but I wanted the alert to 're-appear' as essentially it hadn't been fixed.

This then led me to thinking about any alerts that can move from Active to Ended. It would be useful to configure these is such a way that if they stay Active for a given period, then they re-fire. I understand that is what the multiple thresholds can be used for (especially if the thresholds define time periods) but in this case I may already be at High, there is no further threshold to hit, and I still want to be notified.