Probably because of some rule that you have configured.
See http://www.howto-outlook.com/faq/duplicates.htm

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Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
http://www.howto-outlook.com/
Outlook FAQ, HowTo, Downloads, Add-Ins and more

http://www.msoutlook.info/
Real World Questions, Real World Answers

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"EJ Waits" EJ wrote in message
...
In Outlook, with some senders I received duplicate emails. This doesn't
happen with all senders, but happens every time with some senders. Anyone
have any idea why this happens, and how to fix it? Thanks.