Duplicate E-Mails between K9 Mail & Outlook '03

Both myself and my boss are Android users; I've got an original Droid and he's got the Droid X. We're both running Outlook 2003 at the office, we're both running K9 Mail on our phone, and both our company emails are run through the same POP3 email service. I've had my phone since the week Droid was introduced, and he's had his Droid X for about four months. Nobody has made any changes to any settings (at least, not to my knowledge). K9 Mail is set to leave messages on the server, while Outlook is set to delete the mail from the server as soon as it checks them.

However, the strangest thing started occurring yesterday: when my boss turns on his computer in the morning, any emails that were sitting in his K9 Mail inbox get duplicated when Outlook downloads them. In the duplicate email, the InfoBar in Outlook (that gray status bar that tells you when you replied to or forwarded a message at the top of your emails) just says "Follow Up." He just started having the problem, whereas my phone with it's almost identical system has not.

Has anyone else experienced this problem, or does anyone have any educated guesses as to where I could start troubleshooting this?

Thanks!