If you click on “Alert” in the top menu and then “Alert Rules” you should see the list of alerts currently configured. Click the pen icon (far right) to edit and then set “Max alerts”, “Delay” and “Interval” to your requirements. Max alerts “-1” means it will send an indefinite amount of alerts and then delay and interval should be easy to understand
I’m probably not the best person to answer as I have only been playing with Libre for a little while, but I’ll answer as best I can till someone smarter comes along
The notifications/alerts seem to be time and not instance based, so seeing as Libre polls every 5 minutes (by default), you would simply need to specify a “Delay” of 10 or 11 minutes, meaning a device would have to fail twice (2x5 minutes) before sending an alarm.