Hey @jakelunniss,
The error in Discourse logs seems to be related to another user (note the name attribute).
As for the user who’s unable to log-in, I suspect the issue is that require_activation is set to true in the SSO payload. This prompts Discourse to try and create a new user instead of looking up an existing one. As his account already exists, the error is thrown.
There might be a few reasons why require_activation is true for an existing user, however could you go to the user’s Wordpress profile, check this box and click “Update Profile”
Then ask him to attempt to login again.
(I’m assuming here you don’t have any custom code on your Wordpress which uses the filter discourse_email_verification)
p.s. I would recommend obscuring the email addresses in the screenshots (data privacy etc.)