On a T5 Super Cluster (running 11.2.0.3) I was creating a cascaded standby from an already functional standby using RMAN DUPLICATE and it errored out with

ORA-01671: control file is a backup, cannot make a standby control file

A quick search reveals that it is bug 11715084 that affects most of the 11.x versions except 11.2.0.4. There is a one off patch available for most of the versions or one can install the bundle patch that includes the fix for this patch. I applied BP26 and it worked fine after that.