PGP key signatures that creates a web of trust. Who's to say you are not a clone? DNA also changes over time (although not significantly).

If 100 of your close friends/peers say you are you, then that should be enough...Right? Well...
What if 1000 of someone else's friends say otherwise?

Now it comes down to who has existed first.

If my web-of-trust exists first, I am effectively the real me.