Was discussed several times, think starting point was here https://bitcointalk.org/index.php?topic=366105.0
or maybe here https://nextcoin.org/index.php/topic,471.0.html (but this ends with a link to the above link)

Each time a node sees a transaction it checks if it's the first time a public key used.
If yes, then soft remembers this key, so next user with the same account id will get all transactions rejected.