Problems with duplicate messages

I am using Majordomo 1.94, and have a single list file and configuration.  I have 1200 users subscribed to the list.  I periodically sort the list by domain name to speed up mail delivery.  The problem I am having is that some users complain of duplicate copies of each message, sometimes 2,3 even 4 copies of the same message come through.  I have confirmed that there is no potential for loops in the alias file.  The users complaining are in the list file only once.  I checked my maillog file and for each message that is send via smtp, their e-mail shows up only once.  I am at a total loss!  I have read that smtp servers sometimes have timeout issues and this causes duplicate messages but I would not expect this to be the cause since it happens to the same users all the time.  Does anyone out there have any ideas?
 
Thanks,
Rob