Thanks for the info on editing.  I'll try not to need it!  Also thanks for the cleaning up...

As for identical twins looking alike, firstly, they do not.  They have different fingerprints and different retinograms, and above all, different brains!

Similar? Decidedly.  Eerily similar? Frequently. But identical? Definitely not, and in particular, NOT similar enough to be real replacements for each other, no matter what we might speculate when identical twin brothers marry identical twin sisters.

And in answering the question I assumed that if true identity was not the point, then at least close enough to replace each other would be. I have known eg brilliant sisters with very similar natures and talents, and yet they differed recognisably in personality and performance, though they (in the nicest possible way) wiped the floor with their classmates.

Remember that at the level of some fairly basic reptiles the brain already contains more info than the genes!

Cheers,

Jon