Managing Many Calendars: Checking for Duplicates
When creating appointments in Office 365 or other Exchange systems you can ask ACal to check for duplicates before making each appointment. ACal considers any appointment for the same date and time with the same subject line to be a duplicate. If you are creating timetable appointments for all staff for a whole year you could be creating thousands of individual appointments. Asking ACal to check for duplicates before creating each one of these would slow the process down considerably.
Accordingly, our advice is to create the first batch of appointments without checking for duplicates. If you are updating Term Dates or Events it may be useful to tick the Check for Duplicates box.
If you are updating the timetable during the year ACal offers a better alternative than checking for duplicates. See Updating Timetables
If you find that you have created unwanted duplicates then for a single user Category view provides an easy way to delete them all so you can start again. When managing many calendars see MMC: Deleting Appointments