We run nearly all lists here with X-Tags= Yes.
 
One subscriber (out of 400+) from one of these lists is occasionally
getting duplicate postings.  The claim is that X-Tags= Yes is possibly
the problem.  I have no idea if this is true or not, but I am doubtful.
My questions are:
 
Has anyone else seen a duplicate posting problem that was solved solely
by changing X-Tags?  If not, how would you trace this problem from this
Listserv site to that end site?  Personal mail sent to the remote user
arrives there in single copy...