If you duplicate a database to a higher version, you would restore a RMAN backup and upgrade that. Using duplicate would fail (ORA-39700) since it shall open the auxiliary with resetlogs.

In RMAN 12c, the “NOOPEN” option is available, requesting RMAN to complete the duplicate activity but NOT open the auxiliary database (this would allow for the ‘open upgrade’ option to be manually executed when going between versions).

Source: https://support.oracle.com/epmos/faces/DocumentDisplay?id=2022820.1