There's something inexplicably off-putting about meeting siblings who bear a strong resemblance to each other. It's a weird feeling that's especially amplified in the university setting, most likely due to the combination of enforced intimacy and the sudden detachment from the outside world (as in, you probably won't meet your best friend's teenage brother until graduation, unless he shows up for a weekend, in which case you should force him to binge drink and try to trick a peer into hooking up with him). It's also hard to articulate why it's so odd; we love the idea of siblings who look alike, but it's faintly alienating in practice, like a conversation with a Canadian, or dried pineapple rings.
In Violence and the Sacred, French anthropologist Rene Girard has some funny ideas about why this happens (I'm reading this book for an English course, and I can't stop giggling as I flip through it in the library; anthropology is so hilarious and silly to me). In one of the chapters, Girard attempts to apply his sacrificial theory to ancient Greek tragedy and comes to the conclusion that violence--impure violence, as opposed to the "purifying" violence of sacrifice--is an inevitable product of undifferentiation, a chaotic state that results when hierarchies collapse. In tragedy, this is often symbolized in the conflict between brothers: Consider the section he excerpts from Malinowski's The Father in Primitive Psychology.
"I was then told by my confidential informants that I had committed a breach of custom, that I had perpetrated what is called "taputaki migila," a technical expression referring only to this act, which might be translated: "to-defile-by-comparing-to-a-kinsman-his-face."This is, in many ways, a conception of the tragedy and the breakdown of boundaries that is antithetical to Nietzsche's view of the form as a positive, productive marriage of the Apollonian and the Dionysian that enables us to overcome the real root of suffering, which is individuation (While Girard doesn't discuss Nietzche's theory, he does flippantly accuse him of being a Hellenophile).
Besides being totally hilarious, the idea of taputaki migila--the negative connotations of inter-sibling resemblance--does, in a bizarre, twisted way, begin to explain why the experience makes us uneasy. While noting intense physical similarities between people doesn't drive people to murder them or sacrifice a goat, it does draw their individuality into question. Or, in the case of twins, causes one to constantly worry if they're pulling an old school switcheroo.