Don't you user same storage unit & same volume pool as destination?
If so, you shuld specify other volume pool or other storage unit.
NetBackup System Administrator�s Guide for Windows, Volume I, page 244
NetBackup does not verify in advance that the media ID selected for the duplicate copy is not the same as the media ID of the volume that contains the original backup.
Because of this potential deadlock, specify a different volume pool to ensure a different volume is used.