Reject email if title already used

What’s the proper way to handle incoming emails with the subject equals the topic title already exists?

Such emails are rejected with email_reject_invalid_post_specified. Setting allow duplicate topic titles is disabled. Might it help me if enable?

On one hand, it is good not to mix emails and existing topics especially if duplicate titles not allowed.

On the other hand, for my case (corporate use in isolated network) user expects that his or her email will convert into post in existing topic when subject == title.