The twins, Sam and Eric, are called “Samneric.” The nickname inherently takes away the boys’ individuality. From what we can tell, Jack is the first person to use the Samneric nickname, which is not so much a surprise. Piggy is the only one to take pains to tell them apart. When he’s learning everyone’s name in the beginning, he repeats “Sam” and “Eric” (as separate names), and later asks them to clarify who is who. The distinction doesn’t last long though.
One of the big moments in Lord of the Flies is when Sam and Eric start fighting with each other. Piggy points out that they’ve never done this before, and this is when he (and the reader) feels that things are really getting out of hand. Piggy worries that, if brother is attacking brother, everyone is in real trouble.
Which brings us to an important point: Sam and Eric, like most male twins, happen to be brothers. This notion of “brother turning against brother” can be applied to the boys as a whole. Many of the arguments against war are based on the fact that it is unnatural and wrong for members of the same species to attack and kill one another. Piggy is so disturbed at Sam and Eric’s fighting because they are brothers, but really, we should all be disturbed because, in a way, all men are brothers.