RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of Duplicate Db command at 04/11/2012 14:33:07
RMAN-03015: error occurred in stored script Memory Script
RMAN-04014: startup failed: ORA-09925: Unable to create audit trail file
Linux-x86_64 Error: 2: No such file or directory
Additional information: 9925

for this i had to create the audit_file_dest directory with the directory structure of the primary on the standby side.

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of Duplicate Db command at 04/11/2012 14:31:49
RMAN-05541: no archived logs found in target database

For this I had to do a logswitch on the primary.

SQL> alter system switch logfile;

System altered.

Errors encountered during Creating a Physical Standby using Duplicate from Active Database RMAN command

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of Duplicate Db command at 04/11/2012 14:19:42
RMAN-05557: Target instance not started with server parameter file


I needed to restart the target database using spfile as I was using pfile.