Agree with some of these questions. Maybe some context is needed around the user, and where they are in the process? If it is time-sensitive and they need to add the information right away before they forget then I would say yes.
But also, howHow many users are there on the list? Because if its not long, then maybe it should be disabled so they can see if that user is already added (so it will remove reliance on the duplicate user error and potentially a few extra steps to undo their action, because at what point in the form is the system going to detect duplication? The user effort will be wasted)