Last tested: Sep 2020
In this case a new embedded user would be created. There is no way to tie an embed user created from a script to a regular user. For any embedded user that gets created, the primary key/identifier is the external_user_id field that gets passed in.
Since the embed user will be separate from the regular Looker user, and you can only have one Looker session in your browser, this also means that logging in as the embed user will log you out of your regular Looker session. Users can log in as the embed user in an incognito window if they want to avoid interrupting their existing regular Looker user session.