[simpss]

I have a public/private key-pair from my local government. Comes with my passport and is guaranteed by the gov to represent a single person only.

Although, I wouldn't want to give the public key to google/amazon/facebook/twitter :)

[criley2]

In the twin example, what is to prevent your twin from taking your documentation and receiving a public/private key in your name?

Or simply access that key of yours and use it?

The public/private key only prove you hold the keys, not that you are you.

Not identity, just proves you have access to the keys.