how do i delete duplicate entries in outlook 2011

I use Outlook 2011 for my email and calendar. When I sync it with my iPhone 4 I have duplicate entries. It seems to be the prepopulated dates in the calendars, i.e. Father's Day, Independence Day, etc. I don't know what the pattern is, but I have at least 50 Flag Days on the calendar and the same amount of Christmas, Fourth of July, etc.

How do I get rid of the duplicates and stop it from happening????

I don't want to use iCal, I'm happy with Outlook. I don't sync with Mobile Me.