Comment 1 for bug 2021891

Pod churn is different from cluster start / stop. In the latter case, there is no pod churn, juju is killed (can't run the stop hook), and just running the start hook is reasonable when the cluster comes up again - the same pod as before is just being started up. Whereas for pod churn, juju gets a chance to see the pod is deleted and hence can run the stop hook, and then when a replacement pod comes up, it's treated as new and hence the standard new agent hooks run.

Charm hooks are supposed to be idempotent - the same hook can be run multiple times and the charm should do the right thing. The linked issue seems to imply that start hook running twice starts a second instance. This is a charm hook implementation issue.