To remove duplicated events on your Office 365 Calendar:
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Go to your Calendar in Office 365, then select the Calendar folder from the Navigation Pane.
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Click the View tab, and select List from the Change View drop-down list under the Current View group.
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Click the Subject column heading to arrange all of the calendar items by subject.
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Hold down Ctrl while you click each duplicate calendar item.
When you have selected all of the duplicate calendar items, press Delete.