When I press NEXT, this comes up and won't let me move past it, "This recipientId already exists."

I have seen this posted a few times and only experienced the error once and it seemed to be related to the deletion of Recipient which somehow did not get recognized in the envelope process. My only recommendation is to create a new envelope. If you have put a lot of effort into the envelope and do not want to lose the work, try saving it as a Draft, then continue the Draft to see if the error persists checking the proper Recipients and fields are applied still. If you can reproduce the error consistently I would suggest a DocuSign Support case to have them review the envelope.