From a discussion on the WG call on 2019/12/05 (though it wasn't minuted).

The spec currently reads

When using aria-roledescription, authors SHOULD also ensure that:

1. The element to which aria-roledescription is applied has a valid WAI-ARIA role or has an implicit WAI-ARIA role semantic.
   2. The value of aria-roledescription is not empty or does not contain only whitespace characters.

It seems useful to add that a role description should not duplicate the (implicit or explicit) role of the element since that can negatively impact users, e.g., Braille translation could become more verbose.