If you import calendar items into Outlook that are duplicates of items that already exist in your Calendar folder, and you select the Allow duplicates to be created option in the Import and Export Wizard, you can delete the duplicate calendar items after the import process is complete. Removing the duplicate calendar items is a manual process. The following is the easiest way to do it:

1. In Calendar, in the Navigation Pane, select the Calendar folder.
2. On the View menu, point to Current View, and then click By Category.
3. Click the Subject column heading to arrange all the calendar items by subject.
4. Hold down CTRL while you click each duplicate calendar item in the list.
5. When you have selected all the duplicate calendar items, press DELETE.

 Note   If you do not want duplicate calendar items when you import calendar items into Outlook, select the Do not import duplicates option or the Replace duplicates with items imported option in the Import and Export Wizard.