Idea: Write a service that you pass the unsalted hash to (only salted hashes in the DB please), and the email address and hash type. Stop people if their hash

Idea: Write a service that you pass the unsalted hash to (only salted hashes in the DB please), and the email address and hash type. Stop people if their hash matches any previous ones. Obviously this would stop people providing the same password for all services... But might creep some people out! Would hopefully highlight how insecure/guessable non salted hashes are. Does anyone know best practice for doing things like this?