Following are the possible resolution options:
1) Change the existing external user to be a Google-managed user, ie, add the OpenID domain attribute to the user.
2) Prompt the user admin to delete the newly created Google user from their Google domain, and create a new user with a non-conflicting username.

The following options are not allowed:
1) Keep the existing external user active but disallow the new Google user from logging in to Studio. This is likely to create problems in the future, such as when the external user is converted to a Google-managed user (which will then require user rename).
2) Let the new Google user to take over the existing username even if they are two different people. This can be a data access violation.