Today in JIRA, like mentioned in this documentation, an user who is already an watcher in a ticket will not receive the mentioned you email notification when someone mentions him in a comment. This user will receive an email with the edits instead:

If you mention users who are already watchers of the issue, they receive one email notification only when you save the edits that have their names mentioned.

It would be great if we can have an option to change this behavior in JIRA, since the Mention email is different and calls more attention of the user if he is already receiving updates from the ticket.