I asked for a real HR Case as evidence and now does it turn out to be more complex but still something that needs to be addressed.

To the HR Case used as evidence there is a HR task that were updated 3 times within a short timeframe.

The 3 task updates were added to the HR Case comment field as 3 updates, each update seem to have trigger one "HR Case ......comments added" notification.

3 outbound emails were created within 4 seconds with the same recipient, subject and body so I can understand that the receiver didn't appreciate this.

I start to believe that current weight = 0 (default value) prevent "Ignore duplicates" business rules to skip 2 duplicates.
Do I need to set a WEIGHT value on the Email Notification "HR Case commented" to get this BR to ignore duplicates?