Check your settings in iTunes and iCloud

You might see duplicate calendars if you have more than one method to keep iCloud calendars up to date across your devices. Apple provides two methods: iTunes and iCloud. It's possible to use both methods if you’re using iTunes 10, or iTunes 11.2 or later and OS X Mavericks v10.9.3 or later.

If you have iTunes 10 or iTunes 11.2 or later, check to see if iTunes is syncing calendars:

1. Connect your iOS device to your computer with the USB cable that came with it.
2. Locate your device in iTunes, then select the Info tab. When you select the Sync Calendars checkbox, then iTunes is set to sync your calendars.

Then check to see if iCloud is also updating your calendars: 

• iOS: Tap Settings > iCloud on your iOS device. If Calendars is selected, then iCloud is set to keep calendars up to date.
• OS X: Go to the Apple menu () > System Preferences and select iCloud. If the Calendars checkbox is selected, then iCloud is set to keep calendars up to date.
• Windows: Open iCloud for Windows. If the “Mail, Contacts, Calendars, and Tasks” checkbox is selected, then iCloud is set to keep calendars up to date. 

If iTunes and iCloud are both set to keep calendars up to date, consider turning off calendar syncing in iTunes. Then, only iCloud will update your calendars.

1. Connect your iOS device to your computer with a USB cable.
2. Locate your device in iTunes, then select the Info tab.
3. Deselect Sync Calendars. New calendars and events will appear only once.

If you want to keep using iTunes to update your calendars, turn off iCloud Calendar:

iOS

1. Tap Settings > iCloud on your iOS device.
2. Deselect Calendars.
3. When asked, tap Delete from My [device].

OS X

1. Choose Apple menu () > System Preferences, then click iCloud.
2. Deselect Calendars. On OS X Mountain Lion and Mavericks, when asked, click Delete from My [device].

Windows 

1. Open iCloud for Windows.
2. Deselect the checkbox for Mail, Contacts, Calendars, and Tasks. Then click Apply.

Follow the steps in the next section to hide or delete duplicate calendars. Your old calendars and events will still be duplicated, so you need to either hide or delete them.