@Ping Li Thanks for reaching out. I couldn't reproduce the issue at my end. As per the error it looks like the body is added. Can you please navigate to the code view of your logic app and verify if [body] is added to the body of your Send an email (v2) 2 action which could have caused this behavior for the second and third occurrence of result of your inlince code.
@{body('monthName')}@{body('monthName')[body]}@{body('monthName')[body]}'