EDIT $\ $ There is now an "experimental" SE software revision blocking comments with multiple @name's, see Jeff Atwood's answer below.
As was mentioned in this recent question, only the first @name notification works in comments (besides those recipients notified by default, e.g. the author of the question/answer). However, I recently learned that some folks are explictly using redundant multiple @name's to emphasize to the author that the comment is also explicitly directed at the author - even though the author is always pinged by default, e.g. see the comments here.
This has the following very unfortunate consequence. New users see these multiple @name pings and they assume that this implies that such syntax works generally to ping as many people as they desire. When they don't receive replies from the non-initial pingees (who were not notified) the newbie may make false negative inferences, e.g. my comment wasn't interesting, the pingee isn't sociable, or perhaps even worse. I've encountered this on at least a few occasions, where folks wondered why there was no reply to their never-received ping. No doubt many other cases went unnoticed, leading to who knows how many false negative impressions (alas, comments are not searchable, but perhaps a database dump can be employed?).
Here's another example. Suppose that you posted a difficult problem that has been bugging you your whole life. Nobody answers it. A year later a newbie expert adds a comment to a related question with a link to a paper that completely solves your problem, thinking that there is no need to duplicate the comment in your post, since he pinged you already. But in fact you don't get pinged because you are the second pingee in the comment. As a result, you may never know the solution to one of your favorite problems.
Obviously there are many other analogous problems resulting from folks mistakenly misled to believe that multiple pings work when in fact they don't. Half-implemented and/or broken communication systems lie at the root of many serious problems.
The correct fix would be to have the software warn about such incorrect syntax. But we all know how difficult it is to get the SE folks to make enhancements. Should we instead explicitly discourage use of this misleading syntax, and put a warning in some high-visibility place (faq?).
If folks desire to catch the eye of the author (in addition to the default ping) then why not simply begin the comment addressing the author by name? (without a leading at-sign). This achieves that goal without misleading others into believing that multiple pings work generally.