LISTSERV doesn't care about the 'Errors-To:'  header, which by the way is
a sendmailism, and the duplicate warning is by no means a delivery error.
It does not follow  the same rules as delivery errors,  which are sent to
the list owner and not the original  poster. The duplicate may not be due
to a  network glitch, there are  cases where people legitimately  need to
post exactly the same thing at regular intervals, and without the warning
they might never know that their message is not getting through.
 
  Eric