This happens when you mix wu-imap and vm-pop3d in with the same inbox (ie: if you use squirrelmail, and outlook with pop).. and have also set outlook to leave the message on the server.
wu-imap adds message headers to the emails when you view them, Â which changes the email headers.
Outlook relies on the headers being unchanged to indentify if it's already seen that message.
Since wu-imap changes the headers, outlook thinks it's a new message and downloads it/shows it to you again as a seperate message.
With the "leave message on server" option for pop in outlook, wu-imap is free to change the headers again if you login to squirrelmail, so with pop/outlook, you'll get yet another copy.
Solution 1: use only pop (/webmail), OR only imap (/squirrelmail). Â Note: outlook does support imap instead of pop.
Solution 2: upgrade to dovecot, as it's one program that manages both pop and imap, and the issue doesn't affect it.
Note: If you're already using exim and dovecot and are getting repeated emails, then the issue is more likely related to a segfaulting exim binary.
The solution is to compile exim from source. Related.