The site Brian suggested is owned and operated by another Outlook MVP - me.
Many, many times the information you'll get from outside sources is better
in that it gives workarounds and tips you won't get from any official MS
site.
As for warnings about duplicates, you must of missed this:
Note: You will have duplicates for 2006 and 2007 if holidays for those years
were previously added. Use the By Category view and delete the items in the
Holiday category before importing, to eliminate duplicates.

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Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours
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> Brian:
> Lemme see if I understand this - you are an MS MVP & you are referring
[quoted text clipped - 14 lines]
>>
>> http://www.outlook-tips.net/howto/missinghol.htm
> Lemme see if I understand this - you are an MS MVP & you are referring
> someone to a resource outside Microsoft - interesting.
Not at all. One of the links on the page I cited it the MSKB download for
the holiday update.
> What? Can't tell us that the MS answer is - "Must upgrade to Outlook
> 2007"?
There's no reason. Holiday files are simple text files. Anyone can create
them.
> The link you provided is for holidays for 2006-2012 - it does not
> mention if you will end up with double holidays for 2006 since MS
> previously supplied them thru 2007.
So, just display your calenday in the By Category view, delete the entire
Holiday category and add the holidays again. How hard can it be?

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Brian Tillman [MVP-Outlook]
Outlook 2000 Holiday Update - 30 Dec 2007 19:00 GMT
I tried to add Jewish Religious Holidays and it did not take. In looking at
the txt files I found the problem: The new updated calendar for Jewish
Holidays provides for example "Erev Passover,5763/8/14,8" when the format
should be converted to the CE Calendar like "Passover, 2003/04/17".
Does anyone have a solution to convert the Jewish dates?
> > Lemme see if I understand this - you are an MS MVP & you are referring
> > someone to a resource outside Microsoft - interesting.
[quoted text clipped - 14 lines]
> So, just display your calenday in the By Category view, delete the entire
> Holiday category and add the holidays again. How hard can it be?
Diane Poremsky - 30 Dec 2007 20:38 GMT
Version of Outlook? Did you install the most recent update - and add it to
your calendar? I added the Jewish holidays to my calendar and they appear
to be correct but I'm using the latest version of the holiday file
(C:\Program Files\Microsoft Office\Office12\1033\outlook.hol - change the
Office12 part to your version of outlook) to check it.
http://www.outlook-tips.net/howto/missinghol.htm

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Diane Poremsky [MVP - Outlook]
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> I tried to add Jewish Religious Holidays and it did not take. In looking
> at
[quoted text clipped - 24 lines]
>> So, just display your calenday in the By Category view, delete the entire
>> Holiday category and add the holidays again. How hard can it be?