pop clients getting duplicate emails from a fc4 server

[Christian Motta]

For some reason my clients that use pop to check email  will keep 
downloading the same email, or group of emails, or all email from the 
server over and over again.

I tell clients to keep mail on the server for 30 days - so they can use 
webmail if need be

Most of them use outlook but I have seen it happen to thunderbird too.

I can't figure out is this is a client / dovecot / sendmail issue.

Any ideas?

btw the server is a new fc4 box

thx
-chris