FoafUniqueIdentifiers

* foaf:homepage
* foaf:jabberID
* foaf:mbox
* foaf:mbox_sha1sum
* foaf:weblog

These properties all have the ESW:InverseFunctionalProperty, meaning that they are unique for a given person.

(I wonder, though, if any two people in the world share the same mailbox address...)

They certainly do. Individuals sharing mailboxes is less common in the modern internet, where it's easy to get a free unique mailbox (Hotmail, Yahoo, etc.), but I doubt that it doesn't happen at all.