duplicate loading to outlook express

Hi,

We have a rh7.3 email server, and some users use outlook express to
handle their emails.  They select "Leave a copy of messages on server"
and "Remove from server when deleted from 'Deleted Items' ".   Some of
them complain that sometimes (a few times a month) the mails remained on
the server are reloaded to outlook express as new mails, i.e. duplicated
and marked as new mails (bold).  Then, the user has duplicated sets of
the same mails, one set has flag "read", and one set has flag "unread".

Can anybody help to solve this problem?  Is it a server (sendmail)
problem or a client problem -- when that problem occurrs, it occurrs on
all client machines with outlook express.

I appreciate all help!

Hongwei