Is it possible to duplicate a standby database on the fly?
Step 2: Mount or Open the Source Database Before beginning RMAN
duplication, mount or open the source database it if it is not already
mounted or open. If the source database is open, then archiving must
be enabled. If the source database is not open, and if it is not a
standby database, then it must have been shut down consistently.

Duplicating a Database

To create a standby database with the DUPLICATE command you must
connect as target to the primary database and specify the FOR STANDBY
option. You cannot connect to a standby database and create an
additional standby database.
Basic Concepts of Standby Creation with RMAN