It just so happens that I was on vacation  :-)  at the time of the duplicate
mailings...  I returned to 3400 !! mail files.  (Luckily, they are collected
to disk using GONE... and a colleague was here to increase my disk to
megaspace! ;-) )   I expected 400-500 pieces of mail over the 12-day period.
 
It turns out that only 450 or so were valid of the 3400.  All duplicates
occurred in the May 24-25 time period (dates taken from the Date: field).
I first thought that perhaps the PSU INTERBIT problem caused it, but I also
received duplicate local mail. :-(  The number of duplicates were unpredictable
as well: 2, 6, 11, 12, 20, etc.
 
This is FYI...  I'm not sure if local servers:
 Listserv (1.5o w/FIX15O1) and Mailer (2.03B)
or network servers (anywhere, any level) were guilty or partially guilty.
It was a nightmare!