[qvq]

Identical twins would be a problem. There's no foolproof way of verifying identity today. It might be possible for a system to observe you and generate a set of identifiers. I doubt we can make anything accurate enough to be usable today. Maybe in a decade or two.

[todd8]

Their birthmarks and finger prints are different (out of curiosity I verified this myself with twins that were students of mine) and I believe that their iris scans are different. So a combination of DNA and fingerprints/iris scans should be foolproof.

[zxcvbn4038]

Chimeras are another problem, although very rare.