[Dovecot] quetion about duplicate messages

Hi, we recently switched from Imap-Uw to Dovecot as a pop3 server. I was wondering why people who have the flag 'Leave messages on server' in their email client (Outlook, Outlook Expres) that they receive duplicates of their stored mail? Is it because Dovecot doesn't know that the messages has already been popped by Imap Uw or is the email client which thinks there are new messages on the server and if so..how can I avoid this behaviour?

Thanks in advance for any help.

Kind Regards,

Michiel