User is trying to add a common contact as the client contact for multiple clients, but gets an error that the Display As value is in use.
This happens if you use the same Display As value for the client contact when adding it for another client. Core only allows a unique Display As name for the client contacts even if they are of different clients. Although you can have the same contact details like name, address, phone, etc. for the contacts, you should make sure to use a unique Display As in each case. Check the Core Help for more information on adding client contacts.