Arch backup failing with error ORA-19511

Hi experts,
Arch backups suddenly starts failing in our database with following errors:

ERROR: RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of backup command on ORA_SBT_TAPE_1 channel at 12/23/2011 18:37:47
ORA-19506: failed to create sequential file, name="/rbackup/rmanbackups/ISDDROP_ARCH/ISDDROP_set165_piece1", parms=""
ORA-27028: skgfqcre: sbtbackup returned error
ORA-19511: Error received from media manager layer, error text:
   sbtbackup: file /rbackup/rmanbackups/ISDDROP_ARCH/ISDDROP_set165_piece1 already exists in catalog

Recovery Manager complete.


command that we are using for arch backup as :
RMAN> configure backup optimization on;
2> configure retention policy to recovery window of 10 days;
3> configure controlfile autobackup on;