The furnace.auth.providers.couchdb vocabulary implements an authentication provider which looks up authentication requests in CouchDB. It is necessary to create a view associating usernames with user documents before using this vocabulary; see documentation for couchdb-auth-provider.
Although this implementation guarantees that users with duplicate IDs/emails cannot be created in a single CouchDB database, it provides no such guarantee if you are clustering multiple DBs. In this case, you are responsible for ensuring the uniqueness of users across databases.