[Dovecot] Need help with "mbox sync" problem

[Jim Garrison]

The following is occurring consistently on users in only one vhost
domain.  Other vhost domains are not having the problem.  The email
client in use is the same (in both the working and non-working domains).

dovecot: Jan 29 21:19:18 Error: POP3(user1 at mydomain.tld): mbox sync: UID
inserted in the middle of mailbox /var/mail/vhosts/mydomain.tld/user1
(5629 > 5423, seq=2, idx_msgs=5)
dovecot: Jan 29 21:19:18 Info: POP3(user1 at mydomain.tld): Disconnected:
Logged out top=0/0, retr=95/508506, del=95/95,
 size=506866
dovecot: Jan 29 21:19:52 Error: POP3(user2 at mydomain.tld): mbox sync: UID
inserted in the middle of mailbox /var/mail
/vhosts/mydomain.tld/user2 (5634 > 5629, seq=2, idx_msgs=95)
dovecot: Jan 29 21:19:52 Info: POP3(user2 at mydomain.tld): Disconnected:
Logged out top=0/0, retr=5/69654, del=5/5, si
ze=69562

Once this message starts occurring, dovecot stops deleting retrieved
messages when the user retrieves mail (with 'delete from server' set).
The result is that the entire mailbox contents are fetched each
time, resulting in copious amounts of duplicate messages.

I'm on the rc8 RPM that comes with FC5, but am willing to remove it
and install a later Dovecot built from source.

1) Is this a known problem that can be fixed by upgrading?

2) If not, can someone tell me what this message means exactly, and
   how to gather more information for debugging?
   The MTA is Postfix.

Thanks