I have finally got my email alerting working, and have been testing on disk space. I have configured a Warning at 90% avail and Critical at 95%.
When the server reaches 90% and triggers the Warning, I get two emails for this event at the same time. When it reaches Critical I get one email (as I would expect), then when I clean up space I get one email Warning, and then two emails for return to Normal.
I only have one alert rule being triggered, and only one Email Action (plus the default Logging)