AlertsPreferences.ensure was an unlocked check-then-insert, and a page
load fires the home widgets and the bell together. On kof22 this left
two identical "alerts (bootstrap rule)" subscriptions for the owner, so
every alert notified them twice. First calls are now serialized in the
JVM. Across replicas, each first call keeps the oldest bootstrap
subscription and deletes the rest. Subscriptions a person made are never
touched.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>