>This  would require  more overhead,  I'm sure,  but it  might be  made a
>user-specifiable option (some keyword in the  mail header or text of the
>posting?).
 
More importantly, how is LISTSERV@FOO  supposed to know that LISTSERV@BAR
sent you the same posting for another  list they have down there and that
it knows  basically nothing  about? This  can't be  solved by  having the
servers exchange  this information over  the network: it might  not reach
the destination  in time, actually if  you do 'MAIL XYZ@FOO  ABC@BAR' the
information from LISTSERV@FOO to LISTSERV@BAR  that message X has already
been sent to user Y is  very unlikely to reach LISTSERV@BAR *before* your
original message.
 
  Eric