You can enable persistent alerting to continuously inform you about a critical issue if the issue has not been resolved.

For example, if your server is not responding you will naturally receive a single alert to inform you that the server is not responding. However, with persistent alerting, you have the option to receive X number of alerts during the course of an hour if the server continues to not respond.

Go to Alerts » Alert Settings.
Select the checkbox () next to Enable Persistent Alerting (if not already selected).
Enter the Alert Interval (how often you want to be notified about the issue).
Enter the Alert Duration (the length of time you want to notified about the issue).

If you select Until Cleared, you will continuously be notified until the issue is resolved.
Click OK to save the changes.

You must enable persistent alerting within the alert definition itself to start receiving alerts (Alert Definitions).

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