Oracle rman backup

 Oracle ASM Database is hosted on a Netapp filer and using Netapp snapshot manager to take the snapshot of the disk after queiscing the database. We move this to tape by mounting the snapshot on a different host (proxy), bring up the database in mount mode and backup the database. For this to work, the host should not have any asm disk configured with that group name.

If asm disk group already exists on the proxy host  with the same name and is managed by that asm instance  then how do we mount the asm diskgroup with the same name(mounted thru snapshot)  without dismounting the original asm diskgroup. If we try to add the disk to the existing group using "alter diskgroup data add disk '/dev/raw/raw6' we get the following error