Ensure allow duplicate topic titles site 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).
Expected behavior:
The email contents are appended to the existing topic as a new post (i.e., a reply to the original post in the topic).
Actual behavior:
The email is rejected with the “Title has already been used” error message.
On our forum I’ve set the ability to have duplicate topic titles to on because (similar to @Mittineague) we’re going to have a lot of similar topic titles that are potentially completely different in content.
I think it makes sense for those disallowing duplicate topic titles, but perhaps have a setting to limit the length of time this check is carried out for? Default of 3 months say. If a new post comes in and the existing topic is older than that, it gets created as a new topic. Topic title is altered to something else (no idea what to suggest on that front though).