Technology > Apple Problem Summary. Sometimes duplicate entries will unexplainably appear in the Apple iCal calendar application. This page explains why this might happen, how to clean up the problem, and how to prevent the problem. Problem Cause. Duplicate entries may be caused by any of the following. file corruption in the local synchronization database… Continue reading iCal Removing and Preventing Duplicate Apple MobileMe Synchronizing Calendar Appointment Schedule Event Entries With Archiving Using BusyCal