I checked my Stack Exchange Inbox (drop down list) just now and it mentioned three messages for me. Two were legit - the last was this one on The Tavern which was another Mark.
That follows the standard rules of @blah conflicts, I'm afraid. You would also have got a highlight inside chat for that.
That was a reply; if they had just used the explicit reply (rather than adding the @Mark), it would have known who it meant, but that @Mark was ambiguous so all the "Mark*" who had been active in the room were included.
Usually, auto-complete on the name would have given a less ambiguous match... but they are just "Mark".