The Shadow of the Dead Hand by Fred M. White

Inspector Dwight smiled with the tolerant air of one who listens to the prattling of a little child. The mere idea that the man still lying there, on the floor of the library in Marrion-square, should be anybody but Felix Murdstone was too ridiculous even to argue about. There he was, in his own house, where he had been identified by two footmen, to say nothing of Martin, the butler, who could not possibly be mistaken. And yet, here was this chubby-faced little man with the eyeglass solemnly proclaiming that the body on the floor belonged to somebody else.

"Well," Dwight said. "I have heard some strange statements in my time, but if you will pardon me for saying so, nothing more ridiculous than the remark you have just made. Perhaps you will be good enough to tell me exactly what you mean."