Outlook2000 POP3 duplicates messages

I have a client running Outlook connecting to an Exchange server using a LAN. I have also set up Internet email using POP3 so they can connect offsite using dial-up.
They synchronize their folders upon exiting, but when using offline folders and the dial-up they get mail in their inbox that they have already read earlier that day. Is there an option I have not set or is this just the way POP3 works? If so what is the alternative. They unfortunatley do not want to use Outlook Web Access andbeing my MD I cannot force them to!