I also thought about DNA but like many people mentioned it would be very expensive.
Than I thought about a system that could monitor different parameters (IP, IBAN, Computer, Smartphone, location, language...) that would remain secret. When the system would detect that someone is using 2 wallets it would destroy those 2 wallets. Making it counterproductive claiming more than one wallet. But I don't think such system is possible because it would be very complex and it could give many false positives.
The reason for this thought experiment is that I think wealth is not fairly distributed (duh) and a 'universal coin' could maybe reduce this problem (remember this is a thought experiment). Even with bitcoin (please don't be buthurt) people that join later have a big disadvantage and the biggest reasons for this is are: they didn't hear about it or couldn't access it or the price is to high for them. Giving everyone access to this 'universal coin' would partially solve those problems. Maybe with a staking system they could even have a sort of interest (like a saving account).
DNA is not practical. I would use a combination of personal factors like, voice print, eye print, finger print, and tendency measurement like a finger signature on a smart phone.
If that's not enough for you, you can add additional variables around preference, like allowing the user to pick the order of the sign-in process, further complicating the ability of someone else to compromise security. There are other more complex tendency variables that could be built, but you get the idea.