I will look into the guest user role.
GET on /user/profile only returns information about a user that is already logged in as far as I know. From what I found only /system/user can check every user in the system.
To be more clear, I actually want to make sure the field “first_name” is unique to every user. I’m using that field as a user ID which is displayed to all users of my app. It’s less confusing if people can’t take the same user ID. If this check is missing my app might show 10 different people posting under the same name, but only a DF admin who can see their email would know it’s a different person.