The iPhone includes a calendar application that can sync your data from Google, MobileMe, Outlook and iCal. When you sync your accounts, this can result in the duplication of events on the iPhone's calendar. Both the iPhone and iTunes include options for syncing calendars, so you may find that a calendar gets synced again even if you remove it from the iPhone. If you no longer want these calendars on your phone, you can remove any or all of them from both iTunes and your iPhone.
Tap "Settings" on your iPhone's springboard and tap "Mail, Contacts, Calendars" on the third section of the page.
Tap each account that includes a calendar and tap "Off" next to "Calendars" to remove each calendar.
Press your iPhone's "Home" button to return to the springboard and connect your iPhone to your computer using its USB sync cable.
Open iTunes and click "iPhone" under "Devices" on the left pane.
Click "Info" on your iPhone's "Summary" tab and uncheck the box next to "Sync Calendars With" to prevent your calendars from syncing again.
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