Once, on purpose, I wrote a login system that enforced globally unique passwords. Ie. no two users could share the same password. I can't even remember why it seemed like a good idea while I was designing it, but the problem quickly became apparent once anyone started trying to use it.
Did you store the passwords in cleartext or did you use unsalted hashes? Those are the only ways I can think of to enforce uniqueness, and they're both deprecated practices in themselves.