Steps to reproduce:
allow duplicate topic titlessite setting is off (default).
- (Optional) Email in a new topic with subject *Foo bar baz to the email address associated with either Discourse itself, or in this case, a specific category.
- Read notifications via email and/or replies on the web site.
- Later, add another post to the conversation by emailing the same address in step 2 with the same subject Foo bar baz (rather than by replying to a notification).
- The email contents are appended to the existing topic as a new post (i.e., a reply to the original post in the topic).
- The email is rejected with the “Title has already been used” error message.