When a user is mentioned in the additional comments of a task record, an email notification is triggered to the user as per the OOB email notification - Activity Stream @Mention Email
OOB the email notification is set to trigger whenever a record is inserted.
Release or Environment
The reason being, every time you mention a user in a record, it will create a new entry in the live_notification table.
Now, whenever you insert any record in the database (be it Oracle, MySQL, MariaDB), the inserted and updated time will automatically populate for the record.
So whenever a user is mentioned, a new live_notification record is inserted, with a created time & updated time.
If you have both Inserted & Updated checked under When to send, it will send duplicate emails to the user.
An email is triggered for the created time trigger, and the next email is sent for the updated time trigger.
Uncheck the Updated option under When to send.
Every time you mention a user, a new live_notification record is created.
The same record in the live_notification will not be updated, so there is no purpose of having the Updated option checked.
INFO - The email notification mentioned in this KB does not run on the task table.
The notification is designed for the live_notification table.