Currently, when one downloads and installs current/upcoming calendar
holidays, if there are old holidays already listed by a previous installation
of Microsoft Outlook Holidays, they are not overwritten, they are only added
for a second time. When releasing calendar updates for holidays, only
include updates for the upcoming years. Either that, or have holidays from
previous years be overwritten when installing the new calendar holidays.

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http://www.microsoft.com/office/comm....calendari ng