If you take a look at Microsoft's knowledgebase you soon find out that it lists several situations where Outlook might download the same email message more than once. I listed some of them for your convenience in the next paragraph.
This is by far the easiest way to cope with this situation. The software does not only remove duplicate emails in Microsoft Outlook but also duplicate notes, contacts, calendar entries and tasks making it pretty universal.
- You are prompted for your password, and you may receive duplicate e-mail messages if your profile has two POP3 accounts in Outlook (link)
- Duplicate e-mail messages are downloaded from a POP3 e-mail server account in Outlook 2003 (link)
- Duplicate junk e-mail messages appear in the Junk E-mail folder in Outlook 2003 (link)
This is by far the easiest way to cope with this situation. The software does not only remove duplicate emails in Microsoft Outlook but also duplicate notes, contacts, calendar entries and tasks making it pretty universal.