If you want to sync your phone with a different computer, you should remove the device from iSync on the computer with which you’re currently syncing. (You should sync your phone with only one computer.)
For example, if you’ve added your mobile phone to iSync on your work computer but you want to sync it with your home computer instead, you should first remove the phone from iSync at work. Then you can add the phone to iSync at home and sync there.
Select the device icon in the iSync window, then choose Devices > Remove Device.
If you plan to add this device to iSync on a different computer, the first time you sync you should choose “Erase data on device then sync” from the “For first sync” pop-up menu. If you don’t replace the information on your device with the information on the computer during the first sync, you may see some duplicate information.