This problem seems to be cropping up frequently lately, as we've received numerous contacts from individuals experience duplicate copies of calendars on their iPad.
The problem does not exist on the Mac that the iPad is being synced with, only on the iPad itself.
In every instance, the individuals that were experiencing this duplication were MobileMe customers (trial, subscribers, no matter).
The duplication is caused by having calendar syncing enabled both on the iPad and the iPhone.
1) on your iPad, launch the Settings app
2) go to 'Mail, Contacts and Calendars'
3) turn of OTA (over-the-air) syncing for MobileMe (declining any deletion)
4) re-enable OTA syncing, and confirm the option to merge
You should no longer have duplicate calendars.