RMAN-20246: new db_unique_name is already known to the recovery catalog
the user specified known DB_UNIQUE_NAME value when renaming a DB_UNIQUE_NAME in the recovery catalog.
change the database DB_UNIQUE_NAME to an known DB_UNIQUE_NAME value; or
unregister the new DB_UNIQUE_NAME before renaming an old DB_UNIQUE_NAME
value for a database.
To display the list of all DB_UNIQUE_NAME for the
databases LIST DB_UNIQUE_NAME OF DATABASE command can be executed.