Am I correct that username collisions can only happen within the same realm? ie, if a user is getting a “Cannot complete login for your user with provider ID and username XYZ because the username XYZ is already being used by the user with user ID ”, this would not be caused by them having 2 accounts in 2 different realms?
Usernames are unique per realm so this message means that within the realm there is another user that has the same user name but has a different provider ID. Use the user ID provided in the message to locate that user.