RMAN-05531: a mounted database cannot be duplicated while the datafiles are fuzzy

This error is observed while we are trying active database duplication from standby database while MRP process is still running and applying the changes.

RMAN-00571:====================================================
RMAN-00569:==========ERROR MESSAGE STACK FOLLOWS============
RMAN-00571:====================================================
RMAN-03002: failure of Duplicate Db command at 11/10/2014 17:03:07
RMAN-05501: aborting duplication of target database
RMAN-05531: a mounted database cannot be duplicated while datafiles are fuzzy

Recovery Manager complete

Bug 11715084Active duplicate should work when connected to Standby as source DB 
[ID 11715084.8]

While MRP is running, the datafiles in the standby database get continuously updated and RMAN active database duplication doesn't currently support the standby to be cloned while the standby database is in mounted mode and the datafiles are being continuously updated.This problem is fixed in 11.2.0.4 and on wards releases.

WORKAROUND 

follow these steps
1.Open the standby database(the target) in read only mode.
2.Stop MRP process
3.Proceed with the active database duplication