Does @mentioning users for the first time while editing a comment notify them?

Hi John,

Based on my understanding of "Mentions", a notification is sent to a person the first time you mention them in the content of a page, but not for subsequent mentions of the same person.

If I understand your scenario correctly, you are mentioning an additional user (which I suppose is the first time that person is mentioned), so I believe a notification will be sent to that new user.

Reference - https://confluence.atlassian.com/doc/mentions-251725350.html#:~:text=Confluence%20will%20send%20the%20person,disabled%20the%20user%20mention%20feature).

Hope this helps.