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.
Hope this helps.
Thanks Vinod! I've edited my question to clarify further. Thanks for the docs link as well. It sure does say quite clearly:
A notification is sent every time you mention someone in a page comment or inline comment, not just the first time.
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