Enterprise Manager Cloud Control 13c - Email notification should send only once
What you described is the default setting (for repeated notifications). I guess you have a customized environment and someone configured repeated notifications on purpose.
Repeated notifications can be configured globally in Setup / Notifications / Global Repeat settings, or specifically for metric alerts in Setup / Incidents / Incident Rules / &quot;your rule set&quot; / Rules / Create incident for critical metric alerts / Actions / Repeat Notifications.