Use Thread-Index email header field to add incoming emails as comments on existing tickets instead of creating duplicate tickets
It would be awesome if we could choose to use the Thread-Index email header field on incoming email to reduce duplicate tickets. It happens quite often that people CC our IT ticket queue email address on email conversations, and we get a ticket for every reply-all. If the Thread-Index header field was used, then all of the reply-alls would come in as comments on the ticket created from the first email in that thread, rather than duplicate tickets.

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Jon Saam commented
I've seen other systems use email headers (message id in one particular instance) to identify incoming emails that belong to an already existing ticket.