There are many parts to the spec for the full picture. This here specifies that server MAY issue a new refresh token (clarifying whether it is required), and i

There are many parts to the spec for the full picture. This here specifies that server MAY issue a new refresh token (clarifying whether it is required), and if new refresh token is issued, the client MUST discard the old. I believe the intention here is not to verify whether it was used once as you state. And transient matters (how the client will detect if not received would be a different scope), for example, if client didn't receive (or lost), then likely here you may need restart auth cycle.