How can Community mark a question as a duplicate if he's not a real person? Was it a moderator who marked the question as a duplicate? Was it an anonymous user? Or did something else happen?
When a "duplicate" close vote or flag is cast, a box appears above the question visible to the question's owner. It prompts the author to either edit the question to clarify its non-duplicateness or accept the duplicity by pressing a "yes, that solved my problem" button. If the user chooses that button, the Community user casts the final duplicate vote, thereby saving close vote reviewers the trouble of closing a question that the author knows is a duplicate.