The duplicate messages will have the same Message-ID...
if they post a message they *want* the reply to go to their main inbox as well as the mailing list folder.
Maybe I am just stupid, or maybe my email client is inadequate, but I cannot work out how to filter the same email such that different copies go into different boxes. For me, all 3 or 4 copies of a cross- post get filtered into the same box. I have not yet managed to find a reliably distinguishing set of headers to separate them. As for Reply-to: munging - I agree that _changing_ an existing header would be bad, but would be very happy if mailing lists were to _introduce_ one on messages where none already existed. (I _am_ aware of the general polemic against using a default Reply-To: header in mailing lists, e.g. http://www.unicom.com/pw/reply-to-harmful.html but disagree fundamentally with many of the implicit assumptions and explicit arguments therein.) Regards, Malcolm