Most of the emails I was receiving through Microsoft Outlook 2003 I was getting duplicates. The weird thing was that not every email was duplicated and many times I believed that I had fixed the problem (doing various changes in my outlook settings) but I discovered later when automatic checking occurred that I was wrong.
The problem: I discovered that I have each rule duplicated and that’s why Outlook 2003 was downloading the same email twice.
The fix: I removed the duplicated rules.