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Please take a look at this link and see if the solutions presented help you out:<br />
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<a href="http://social.technet.microsoft.com/Forums/office/en-US/e71a43b3-085a-4434-857e-590792bdb42e/duplicate-calendars-in-outlook-2010-after-migration-from-outlook-2003-using-usmt-41?forum=officesetupdeployprevious" rel="ugc nofollow">http://social.technet.microsoft.com/Forums/office/en-US/e71a43b3-085a-4434-857e-590792bdb42e/duplicate-calendars-in-outlook-2010-after-migration-from-outlook-2003-using-usmt-41?forum=officesetupdeployprevious</a>
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After upgrade to exchange 2013 from Exchange 2010 30% off all users have 2 or more calendar inn Outlook 2010 .<br />
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if the have calendar inn exchange 2010 they get kalender (Norway) and calendar in exchange2013.<br />
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If the have Kalender(Norway) inn exchange 2010 the get calendar and Kalender in exchange 2013
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After upgrade to exchange 2013 from Exchange 2010 30% off all users have 2 or more calendar inn Outlook 2010 .
 
if the have calendar inn exchange 2010 they get kalender (Norway) and calendar in exchange2013.

If the have Kalender(Norway) inn exchange 2010 the get calendar and Kalender in exchange 2013

Exchange2013 upgrade from Echange 2010  calendar problems

The Microsoft Legacy Operating System topic includes legacy versions of Microsoft operating systems prior to Windows 2000: All versions of MS-DOS and other versions developed for specific manufacturers and Windows 3/3.1, Windows 95 and Windows 98, plus any other Windows-related versions, and Windows Mobile.