I think you should take a look at the LISTSERV Support FAQ, question #16,
for our take on duplicates.

 http://www.lsoft.com/lsv-faq.html#Sect16

I also think your user is dreaming :) at least about the business of
LISTSERV sending three identical copies at the same time to three relay
hosts in the hope that at least one copy will go through (at least that's
the way I read it).  Also, unless you have configured things differently,
mail from LISTSERV is bundled together into batch SMTP jobs so if there
are multiple recipients at a single host, they all go together in a single
job and are "exploded" by the remote host into the appropriate mailboxes.
The bottom line is that your user is incorrect as to his thesis of duplicate
mail.

Nathan