This parameter defines the number of copies of each save set created by RMAN with an external backup library, that is, backup_dev_type = rman_util|rman_disk|rman_stage.
Syntax: rman_copies = 0|1|2|3|4
Default: 0 (one copy, same as 1)
This parameter was previously called rman_duplex.
See also:
RMAN Backup with an External Backup Library
Oracle documentation