If I have duplicate users with the same name. user22 is retired and 
user24 is active. Trying to restore user22 reports:

  roundup> restore user22
  Error: no such class "user"

  Usage: restore designator[,designator]*
        Restore the retired node specified by designator.

        A designator is a classname and a nodeid concatenated,
        eg. bug1, user10, ...

        The given nodes will become available for users again.

It should report that there is already a user with the same key
value (username in this case) that is active.