When migrating from Single Sign On -> Azure AD to AuthHub Azure AD you may encounter error "Domain already exists" after authorizing the application deployment within Azure AD (Entra ID).
To resolve this error, please navigate back to Setup -> Admin -> Single Sign On, and select "Unconfigure SSO".
This will remove the existing connection for Single Sign On, and allow you to redeploy SSO using the new AuthHub Azure AD method and complete the migration.
Note: If you are a Partner, a customer user account that is an Administrator must perform the configuration of AuthHub Azure AD within their tenant from their own account.
The Azure AD and AuthHub Azure AD setup process claims the domain of the user performing the configuration. Meaning if a Partner user performs the setup their Partner User Account domain will be claimed against the Customer tenant.