As discussed on https://github.com/ezsystems/ezpublish/pull/428 it makes more sense to have a unique key for ezuser.login than just a simple key. However it causes problem when 2 user are registering because both will have the empty string as login.

An alternative would be to register the user with a temporary login based on the contentobject_id which is itself unique. e.g.: newuser_##objid##