I recently approved an edit to Zeroes of characters of general linear group induced from certain characters of parabolic subgroups by user @math seeker. I approved the edit, which was otherwise excessive, because I thought it was by the same person who had asked the original question, @mathseeker. (It struck me as strange that such an edit wouldn't be automatically allowed, but I thought maybe new users had a different experience.)
Having discovered that these users were potentially different, I left a comment to the editor letting them know that, if they were not the same as the author of the original post, then the edit was excessive.
All that background is not the question. The question is as in the title: how, if at all, can one
@-notify exactly one of two users whose names differ only by whitespace? (In this case it doesn't matter, since one not only need not but can not
@-notify the asker, but the question makes sense in other circumstances.) I guess the question can be asked more generally, e.g., for users that have identical names, not just identical-modulo-whitespace names.