/acs-admin/users/user-add pre-generates the user_id for the new account, so if you're getting there by hitting the back button, or if you're behind a cache, you may getting a stale "new" user_id. ACS seems to silently ignore the error when it attempts to create the new user in order to nicely handle the "user hit the submit button twice" case.
If you go to /acs-admin/users/user-add and force-reload the page, you should be right...
There have been a few threads about problems with OpenACS and caching lately - perhaps it would be worth changing the default master templates to make OpenACS less cacheable by default...
