The tenth commandment of Detective fiction is:
Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
In his 1939 commentary on his decalogue, Fr. Knox said:
The dodge is too easy, and the supposition too improbable. I would add as a rider, that no criminal should be credited with exceptional powers of disguise unless we have had fair warning that he or she was accustomed to making up for the stage. How admirably is this indicated, for example, in Trent’s Last Case!
A few of these commandments have, over the years, become less applicable simply because people have developed the good sense to not violate them. I think that this commandment may be the one for which that is most the case. I can’t think of a story I’ve read—good or bad—in which twins and other doubles appear.
Well, that’s not quite true. There’s an episode of Scooby Doo where a woman was being framed as a witch by her (unknown) twin sister. And there was a Poirot where a murderer established her alibi by having a famous impersonator pretend to be her at a dinner party—but that certainly follows the commandment since the main thing we know about the impersonator is that she was extraordinarily skilled at pretending to be other people. But those are the only two examples which come to mind.
I should note that I’m thinking about really skillful disguises, where a person can interact with others, in person, for quite some time, and be taken to be someone else who wasn’t really there. Minor disguise, by contrast, is a fairly common device in mysteries. It’s a time-honored tradition to have the murderer pretend to be the victim so as to fake the time of death to a later time for which the murderer has an alibi. So much so that these days if a person overhears a conversation the victim was having through a closed door, or saw the victim doing something but at a great distance and with his face obscured but you could tell it was him because of the bright red scarf he always wore, one’s first thought is that it was the murderer pretending to be the victim. In such a case, woe to anyone who has an alibi for the time the murder is supposed to have happened.
With regard to twins, Fr. Knox’s commentary is interesting: “The dodge is too easy, and the supposition too improbable.” These are two different objections, and not particularly related to each other, though I think the conjunction is important here.
The first objection—that the dodge is too easy—is interesting because it is in a sense the essence of a twist that it is something which explains a lot once you know it. But this is not an intellectual twist; it is, rather, a natural twist. It is an oddity of nature that there should be such things as identical twins. And it is the essence of a mystery that the thing unraveled should have been twisted by the hand of man, not of God. It is legitimate to try to understand the mysteries of God, but it is a very different book in which that is done.
The second objection—the supposition is too improbable—is also interesting because it is the heart and soul of a mystery that the obvious solution is not the correct solution. And twins are not that uncommon. According to the statistics I found when googling, about 1 in 250 births is of identical twins. It’s possible that it’s a little less common in England, but this is not so uncommon that no one would think of it. It’s not nearly as esoteric as, say, a poison which hasn’t been discovered by science yet.
I think it’s the combination of being uncommon and explaining everything which makes it unfair. It’s not the sort of thing so likely that anyone in the story will do anything to rule it out, and it certainly will explain away just about anything inconvenient in the story. As such it’s a perennial possibility that the reader has no good way to rule out. That being the case, it should be ruled out as a matter of course and positive hints as to its possibility included if one is going to go down that route.