There really is something big about the biology you're missing here: the human brain doesn't stop its development at birth(or conception, for that matter), so depending on their upbringing, two twins can have very different brain structure - that's why they can end up with very different personalities.quetzalcoatl9 said:im assuming that by this you are speaking in terms of biology, and not referring to each person being "special" or having a "soul" or anything like that.
so we are then speaking of two organisms that are biologically identical, aside from any minor biological distinctiveness that may occur through interaction with the environment. e.g. one twin may develop dark spots on their skin differently from the other due to oxidation damage and so forth - this is ok, and according to my definition does not make them less identical biologically.
of course it is obvious that they will have different environmental experiences, and so they will not have the same memories or perspectives. but again, this does not detract from the fact that they are biologically identical.
so: i would consider twins to be two instances of the same person that coexist in the same spatial and time domain.
conceptually: wipe out my memories and then send me back in time to interact with myself - and you would have twins. no less the same person!