Click on the Duty's exit (or go into the menu and leave), and get prompted with something like this (obviously, you don't do Doma Castle with 8 people. It's a quick photoshop to get the point across):

If you are worried about "shaming", then disable chat when this window is open. Or simply prompt it during the black screen after the duty.

Would be a good start to provide people with a little tool to help them compare themselves with others.