Half or more of the Alertmanager instances within the same cluster are crashlooping.
Alerts could be notified multiple time unless pods are crashing to fast and no alerts can be sent.
kubectl get pod -l app=alertmanager NAMESPACE NAME READY STATUS RESTARTS AGE default alertmanager-main-0 1/2 CrashLoopBackOff 37107 2d default alertmanager-main-1 2/2 Running 0 43d default alertmanager-main-2 2/2 Running 0 43d
Find the root cause by looking to events for a given pod/deployement
kubectl get events --field-selector involvedObject.name=alertmanager-main-0
Make sure pods have enough resources (CPU, MEM) to work correctly.