On a T5 Super Cluster (running 18.104.22.168) 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 22.214.171.124. 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.