This seems to be a know "By Design!" issue and disabling "Turn on shared calendar improvements" seems to fix this..
"Unable to add a shared calendar more than once to the Calendar module
After enabling Shared Calendar Improvements, you are not able to add a shared calendar more than once to the Calendar module. With the MAPI protocol it was possible to add the calendar more than once by adding it to a different calendar group.
At the time we tested this functionality in Current Channel, attempting to add the same shared calendar twice to a different calendar module, (Add Calendar, From Address Book) or (Add Calendar, Open Shared Calendar), opens the calendar checked in the calendar pane. This was the behavior if only the primary calendar was open in the Calendar module.
By Design
At the time this item was added it was a design limit and is not currently scheduled to be changed."
See Known issues with Outlook Desktop Shared Calendar Improvements - Microsoft Support
and
How to enable and disable the Outlook calendar sharing updates - Microsoft Support