It is theoretically (ie. cryptographically) possible to limit quantity
without compromising anonymity. The centralized solution would be to create
a specialized currency, call it person-coin (PSC), and have a central
authority give everyone 1 PSC. People can then use decentralized mixers,
Zerocoin, etc, to mix up their PSC, and then to use your unique identity
you sign a message with the address owning the PSC. You can recover a
public key from a signature, so any kind of online service would simply
maintain a list of all public keys to check for duplicates.