Woo dog. This would help us a bunch.

For clarification:
The request is to match message-id as if it was an in-reply-to header with itself. If the message-id already exists, thread it as a reply rather than a new message.

Doesn't this already happen? We've definitely seen customers 'cc' a message to multiple departments and all end up as a single ticket with the other messages threaded on it.... though how that particular one is chosen and how best to handle that scenario in terms of workflow is a bit tricky.