Hi @Greg Yvkoff Gotcha - It's still a bit open-ended for me to pin the issue to a specific problem. I attempted to reproduce this on an existing APIM instance that I had running and did run into an issue at the "Complete Sign Up" page where after filling out the fields and submitting the form, I was met with the same "User with a specified identity already exists" error and not a success message. Ignoring the error message, I immediately navigated to the Sign-in page and was able to sign in successfully with my AAD account. Prior to getting here, I also noticed that I had an incorrect Redirect URL specified to my AAD App Registration which was causing the OAuth flow to fail because APIM provides separate Redirect URLs for Legacy and the new developer portals.
I'm going to start from scratch again with completely a new APIM instance shortly in an effort to identify any gaps that might exist in our documentation. If I don't find any faulty/missing step in the doc and if you also confirm that all of the steps were correctly followed in your setup, I would isolate the issue as something specific to your APIM instance and proceed to request you to send us an email with more info for identifying the root cause in 1:1 setting with you. Hope that's OK and please stay tuned for my next update soon.
UPDATE: APIM team confirmed that, for admin accounts, users have to log in to the Developer Portal via Azure Portal.