Can I add emails with the same title of an existing ticket as a comment instead of creating a new ticket?

Question

Is it possible to have an email with the same subject line as an existing ticket threaded as a comment instead of generating a new ticket? 

Answer

No, it is not possible to configure this workflow. Emails must have matching message-IDs in order to thread into existing tickets. This logic is hard-coded and cannot be altered. 

For more information, see the article: Why do emails thread to the wrong ticket?