When RealtimeNotificationGenerator listener is triggered, it doesn't check whether event is local or remote. This leads to situation when notification (i.e. email) is sent from every node in cluster.
Although it's similar to XWIKI-6235 (which is more general and considers Scheduler more), solution to solve this is quite easy.
There are two solutions how to solve this, because watchlist doesn't need to be enabled on every node.
1) Drop events from remote sources
2) Drop events from remote sources only if watchlist.realtime.allow_remote is true (allows administrators to run watchlist only on one node -> one source of connections to SMTP server)