You already have duplicated calendars in iCloud
iCal allows you to have duplicated calendars. When you enable iCloud, it merges your existing iCal calendars with your iCloud calendars and this can cause duplicated calendars. In this case, remove any duplicated calendars from iCal before syncing with SmartPad (iPad) or SmartDay (Mac).
You are syncing iCal to more than one device
If you have SmartPad on your iPad, and SmartDay on your Mac, you may have a ‘sync loop’ which is created by syncing SmartPad and SmartDay to mySmartDay while you also sync iCal on your Mac to your iPad (and iPhone) calendars via iCloud, iTunes or Exchange. Do fix this, either disable SmartPad from syncing with the iPad calendar, or use the “Remove suspected duplicates” feature of SmartPad. See our mySmartDay synchronization wizard this link for more info.
You have an old “Mobile Me” account
If you have an old “Mobile Me” account that has not yet been deleted from your iPad and Mac “Calendar” settings, delete it. Presto! No duplicated calendars!