duplicating a relayd load balancer monitor in reality just overwrites the existing one and renames it.
Create a load balancing monitor called "Xhttps" and configure it to monitor https. Further configure it to use a hostname 'www.google.com'.
Now create a pool that uses this monitor.
Finally, click the "+" to duplicate Xhttps monitor from the Monitors tab, and call the new one Yhttps and set the hostname to "www.yahoo.com".
Go check your pool. It is now using the Yhttps monitor, and the Xhttps monitor is disappeared.
Basically, the duplicate button is equivalent to the edit button.