You will you receive the following message if you previously added the holidays for a specific country.
However, if you are upgrading Outlook or using an updated HOL file, it may be detecting older holidays.
Microsoft Outlook 2007's holiday list included dates through 2012.
You can download the Microsoft installation file here.
In Outlook 2007 and Outlook 2003: switch to the By Category view. Select the Locations you want to install and complete the wizard.
In Outlook 20: switch to a List view then arrange by Category. While its very easy to remove duplicate Holidays and a duplicate removal utility is not necessary, if you have duplicates of other calendar appointments, one of the tools below can remove the duplicates.
Microsoft Outlook comes with a file named (in Outlook 2000 and older) that includes lists of holidays that you can import into your personal Calendar folder using the Tools | Options | Calendar (or Calendar Options) | Add Holidays command.
This makes it easy to add holidays and many users wind up importing the same group of holidays twice.