99.99 times out of 100, they are receiving duplicates not because of
LISTSERV processing, but because the mail service between the client and
server is having problems.

That having been said, sometimes subscribers are on the list twice:
subscribed (and autoforwarding) from an old address, as well as a new address.

Most lists are setup to check for duplicate postings from being generated
thru the software.

As always, two complete message (headers) as received by the complainer
are required for accurate analysis.

/Pete Weiss @ Penn State