First, sorry for delay
After you add a diskless M/D and you specify MAC address, you must click on "Setup Diskless Media Directors", wait for the setup process to complete, then do a ‘quick reload router’, and then you can bootup your new diskless media director.
We released 18.104.22.168 and i just tested on our test machines addind M/D's and there are no problems.
If you didn't follow up these steps please try again.
Related to duplicate entries, I can't repro here this problem.
you can also try to upgrade your system to 22.214.171.124 and check if the problem persists.