The "login" for Users is generally their email address.

You are getting this error because there is already a User record in Odoo with Email set to the same value as the one you are updating.

Check both ARCHIVED Users and PORTAL Users to find which one it is.  

If it is an Archived User, you can change the archived User to a different email (or put _old at the end) so you can now save your User record.

If it is Portal User, you may want to consider just changing the User type to a full user, or changing the email address to something else - whatever suits your needs.

Users can only have one login to Odoo.

More at https://www.odoo.com/documentation/15.0/applications/general/users/manage_users.html