I often receive duplicate emails when somebody replies to me or to a
thread containing me. I assume this is because of use of the "Reply All"
button, where "Reply List" should be used (or "Reply" if your client
lacks it, but setting the To: field to internals).
Is this the reason? If so, I'd suggest to people who are guilty of it to
use their client's Reply List feature. It saves the environment by
avoiding duplicate emails :P