Alejandro Vargas' Blog

At this moment renaming an ASM Disk Group is not supported.

Available commands to manipulate Disk Groups are : Create, Alter, Drop, Mount, Dismount and Rebalance.

An enhancement request was filed on February 8, 2007, to provide this functionality.
Current status of the request is : command 'renamedg' will be implemented in a future release.

• If you are cloning a database based on ASM and you wish to create several instances from the clone on the same server, you will need to rename the Disk Groups to be able to mount their copies, i.e. Production database disk groups are PRDDATADG and PRDARCHDG, and you would like to mount them from an EMC copy. That would not be possible because of the same disk group names, ASM does read the metadata on the disk headers and find that these disks are already mounted, then returns "ORA-15020: The discovery encountered two disks claiming to be the same named ASM disk".
  
  
• If you have several production databases using generic Disk Group names, like DATADG and ARCHDG and you would like to mount them on a test or development server the same problem will happen.

• If the problem is related to several production databases that have the same disk groups names, and you want to mount them on a development or test server, then you may implement a disk group naming policy that assure disk group name uniqueness, i.e.  <SID><DATATYPE>DG this will produce names like CRMDATADG, APPLDATADG.
  
  
• If the problem is related to cloning databases on the same server you may use Rman duplicate database feature. This option do require an Rman backup, you may use the compressed backupset  option, the compression rate is very high, about 80%.
  
  A step by step example to database cloning using Rman can be found on this post