Unfortunately, I don't think we can actually make them UNIQUE on the backend. The following is valid in the hyperdb:

Make an object A with key "foo".
Retire object A.
Make an object B with key "foo".

If we enforced UNIQUE-ness at the SQL level this would fail because there would be two keys "foo".

That said, the hyperdb is supposed to enforce its own notion of uniqueness. A quick code inspection shows the create_inner for the backends check to see if the key is in use and raises an exception if you try to create a duplicate. I tried creating two users with the same username from the web interface and received the expected error.

If you were able to create duplicate keys from normal roundup usage we should investigate that further to figure out how and why. I don't suppose there is a reproduction scenario available for this?

This is pretty old so maybe something was fixed in roundup in the interim. I'll keep this open for a few days in case there is further input and then close it as Won't Fix or Works For Me.