RMAN Skip Previously backed up files

1. Configure the rman backup optimization to on
RMAN> configure backup optimization on;

new RMAN configuration parameters:
CONFIGURE BACKUP OPTIMIZATION ON;
new RMAN configuration parameters are successfully stored

2. backup database to skip already backed up files.

RMAN> backup database;

Starting backup at 13-MAY-14
using channel ORA_DISK_1
skipping datafile 2; already backed up 2 time(s)
channel ORA_DISK_1: starting compressed full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00001 name=+DATA/cvrman/datafile/system.256.847354793
input datafile file number=00005 name=+DATA/cvrman/datafile/example.dbf
input datafile file number=00003 name=+DATA/cvrman/datafile/undotbs1.258.847354795
input datafile file number=00006 name=+DATA/cvrman/datafile/tbs1.294.847433665
input datafile file number=00004 name=+DATA/cvrman/datafile/users.259.847354795

3.Override the optimization by using
backup database force