Oracle 11gR2, duplicating and refreshing cloned database
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.