An email address can only be associated to a single GitHub account.
If “Alice” adds alice@example.test to their account, GitHub will check if the email is not associated to an existing account, and then proceed to handle the request. Then, when “Bob” tries to add the same email to their account, GitHub prints the following error message “ Error adding alice@example.test: email is already in use.”
In your example, GitHub will display the username of the first user to add the (unverified) email to their account.
If “Alice” adds alice@example.test to their account, GitHub will check if the email is not associated to an existing account, and then proceed to handle the request. Then, when “Bob” tries to add the same email to their account, GitHub prints the following error message “ Error adding alice@example.test: email is already in use.”
In your example, GitHub will display the username of the first user to add the (unverified) email to their account.