There was this evolutionary biologist by the name of William Donald Hamilton, aka Bill, and he was a pretty big deal. He was a member of the Royal Society, and he studied altruism and kin selection, among other things. His work is especially interesting when you look at genetically identical individuals. You will have to forgive me; I am a science nerd.
Some species, like bees and ants, have one reproductively active female (the queen) and many sterile females (workers). These workers take care of offspring and generally... do work, as the name suggests, leaving the queen plenty of free time to make more babies. It had long bothered biologists that this seems contrary to natural selection, survival of the fittest. Being unable to reproduce makes you pretty unfit, in natural selection's mind. (Remember, we're talking about insects, not people, don't shoot!). Darwin himself pondered this problem. Hamilton found a formula describing how being unable to reproduce might be beneficial for the overall fitness of the individual's kin.
Hamilton's rule: rB > C, where r = relatedness factor, B = benefits to the reproductive recipient of such altruism, and C = costs to the individual in terms of fitness (fecundity).
This is where we come back to the subject of twins. "r" is how related you are to someone else. You are 0.5 related to a parent and a full sibling. You share half your genes. You are 0.25 related to a half-sibling, because that half is halved again. You are 1.0 related to yourself. Oh yeah, AND your clone, if you are the product of an in utero cloning event.
Using Hamilton's rule, if I didn't reproduce and I instead helped Baby A to reach her full fitness (helping her raise children, giving her the opportunity to have more children, etc.), I would still be pretty darn successful. As successful, in fact, as if I had gone and had my own children. My genes are her genes, and therefore I am pretty fit if she reproduces and I don't. There is practically no cost associated with me refraining from having offspring to help my sister, from an evolutionary standpoint. My genes are still passed on to the next generation.
Of course, humans cannot be entirely compared to the order Hymanoptera. But this was an interesting thing to think about during biology today.
Oh! AND my professor actually mentioned identical twins/clones today for once. Well, didn't actually mention, but it was on one of the slides. Score!
PS: Yes, all those links are to Wikipedia. I love me some wiki.