Double calendar entries?

I just got a new iPad. After going through setup and sync, I'm getting double entries in iCal as it appears on my iMac and the new iPad. If I use iCloud on my browser, I have single entries. Any ideas on my problem? I must have messed up something on one device or another, but I can't figure it out. The iMac is 3.06 GHz Intel Core i3 with 4GB 1333 MHz RAM, running Lion 10.7.4. The iPad is a 16G model with Verizon 4G, v. 5.1.1. Thanks very much!