If not, it should be. 8)
Spoiler zone for s3! and very probably s4, particularly if you aren't up on ACD canon. Also the pastiches.
Mycroft and the end of s3 ep3. Yes. This is what I believe too, by the way.
Because really, two fakes... not so probable. It'd be tacky. I can't imagine JM lacking class in that way; if you're gonna box your nemesis into a corner, it's gotta be REAL.
It is not the same as "painted into a corner" because the way to solve that little problem is to let the paint dry, in the words of a fictional character.
(Doesn't Ben look very sad as SH in that pic set? Awwww!)
And along those lines, another part of canon is The Other M---. Because there were at least two and possibly three of 'em.
Maybe that's what drove ours batty. Having one or more siblings with your name could definitely cause serious identity issues, let alone sanity problems. No, really.
Pastiche land, or: See, fan fic really HAS been written about the Sherlock Holmes universe for decades! And published too!
I think having three JMs is out of canon, from Enter the Lion, which was a novel written supposedly from Mycroft's point of view. I enjoyed the book.
But this little addition to Sherlockiana might've been from one of Nicholas Meyers' SH novels. I can't remember. I do know that I read The Seven-Per-Cent Solution and possibly the book that followed it. You should too.