I configured an alert to watch failed payment in every 15 minutes and set at most every 15 minutes.
When failed payment happened, we received an alert, that is we want, but 15 minutes later we received the second alert caused by TRIGGERED to OK.
For non-technical personnel, the second alert is incomprehensible
So, how can I mute the second alert and maybe my alert settings is wrong

Alerts are designed to send both alerts. Sending alerts only for TRIGGERED events is not supported.

From a cursory look it should be easy enough to implement a mute on the OK notifications. We’re happy to review a pull request implementing this as a setting for individual alerts.