Assume I've cloned PROD database to TEST with RMAN "duplicate target database to CLONE from active database;". After several days I need to refresh CLONE. Will RMAN transfer all the datafiles from PROD to CLONE or only datafiles with changed data if I will issue "duplicate from active database again" ?
It will transfer all datafiles again.
The previous duplicate already changed all your datafiles. You have a new database incarnation, with a new name, a new dbid, which was written in the datafile headers.
RMAN duplicate skips existing datafiles in case of re-running a failed duplicate so it does not perform the same work again, but that is a different use case.