Reproduce:

• a user username has admin access to a website,
• a user another-username also with admin access to the same website,
• login as another-username
• go to Admin > Users
• then in the input box add admin access to usernaME

then:

• Got: username lost admin access.
• Expected instead: This user has access to this website already. (ideally, since Email in login data should be case insensitive. #8548 Case-insensitive login #8610) or User usernaME doesn't exist.

follows up #9379