Most people have 'I always want copies of my own messages' checked in their profile. This is mainly for Gmail (and some other emailers) that would hide list messages sent from ourselves. For messages sent by people where that is checked, we'd change the Message-ID field for the message to something new before we stored the message in the system, and before we sent the message out to everyone. So everyone would get the new Message-ID.
I've just changed it so that only the original sender will now get a message with a new Message-ID. Everyone else will get the original Message-ID. This changed was requested because of in some scenarios involving cross-posting between lists and CCing messages, the old way would cause duplicate emails.
I've tested this and it should not result in any threading issues. But please let me know if you see anything strange.