When you set up an iPhone or iPod touch with iPhone OS 3.0 or later and MobileMe, a dialog may ask you to merge your contacts or calendars with data already on your device. If you choose not to merge, your MobileMe data will be kept separate from the other data on your device; this may cause redundant information (for example, multiple copies of the same contact).

You may also see duplicates if you are syncing data with MobileMe as well as syncing that information through iTunes when your iPhone/iPod touch is connected to your computer by the USB cable.

To confirm you are in this state, while viewing Contacts/Calendars on your iPhone/iPod touch, tap Groups/Calendars in the upper left corner. If you see “On my iPhone/iPod touch” heading, as well as a heading for your MobileMe account, your data is being stored multiple times on the device, causing it to appear as duplicates.

If this occurs, and you do not want your data to be stored separately on your iPhone/iPod touch, first make sure that you have disabled syncing of Contacts/Calendars to your iPhone/iPod touch in iTunes. Connect your iPhone/iPod touch, select it in iTunes, click the Info tab, and disable syncing of contacts in Calendars. After this is complete, turn off MobileMe syncing for the affected data type (such as Calendars):

1. Choose Settings > Mail, Contacts, Calendars, then tap your account, and slide to Off.
2. When prompted, select Delete from My iPhone/iPod touch.

Now you need to re-enable syncing:

3. Choose Settings > Mail, Contacts, Calendars, then tap your account to slide the data type on.
4. When prompted, select Merge with MobileMe