About ten years ago, I read an old short story about Sherlock Holmes, which was not one of the 'canon' Doyle stories. I think it may have been written by one of the well-known later detective fiction authors, such as Agatha Christie or P.D. James, but I'm not certain.
It was shamelessly based on the canon story "A Case of Identity" - I think even the title was similar. But unlike the original, the spinoff features Mycroft Holmes together with his brother and Watson. The facts of a case are presented to the trio, just as they are to Holmes and Watson in "A Case of Identity", and then each man makes his own assessment of the case.
Sherlock Holmes goes first, and describes the conclusions he has drawn and what he thinks the solution is (this may have been similar to the solution of "A Case of Identity"). Then Mycroft contradicts him, describing an entirely different solution of the case. Finally, Dr. Watson speaks up and says that both of the Holmes brothers are wrong; he gives a third solution, even more different from the other two. At the end of the story, it is confirmed that Watson's explanation was the correct one.
Has anyone else read this story, or can point me to its title or author?