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$\begingroup$The system using at-sign is a potential source for spamming individuals with irrelevant messages. To limit its abuse certain restrictions are in place, with the single user being one of them. Perhaps someone will provide a helpful link to documentation about this system. Gerhard "This Is Where It's At" Paseman, 2020.05.03.$\endgroup$
$\begingroup$Just to clarify - your preference would be to have the situation before this change? (I.e., multiple @username is allowed, but only the first one creates the notification.) Or your preference would be to having possibility to ping several users by the same comment?$\endgroup$
@username
pings in comments? $\endgroup$