The synchronization features of iCal are exceptionally useful for keeping calendars in sync on different computers and mobile devices; however, there may be times when problems with the synchronization occur, resulting in more than one copy of the same event being distributed to all devices. Sometimes this happens only with one device, in which case the fix can simply be resetting and resyncing that device; however, at other times it may be impossible to identify the root of the problem.
In this case, the easiest solution is to reset the calendar synchronization data on all but one master device, set that device up according to your preferences, and then sync it back to the remaining devices. Here is a step-by-step procedure for how to do this in a way that will ensure that your calendars are set up without any residual metadata or configuration options that could be contributing to the problem:
Log out of MobileMe
Go to the MobileMe system preferences and click the "Sign Out" button on all Macs you sync with. Doing this will ensure that nothing gets synced when you clear or alter your calendars.
Clear out iCal
Go to all but one device (the one you will use as a master device) and remove all calendars from them. Make sure they are completely empty, then set them aside.
Set up master calendar
On the "Master" device, remove all duplicate calendars and clean up iCal as best you can, until you have it the way you want it.
Back up the system (Time Machine) once iCal looks the way you want it
With the calendars set up, be sure to back up your system using Time Machine, so the current calendar configuration is saved in a restorable state.
Export all calendars individually and save them as .ics files in a safe location
Now that the calendars are backed up, export them from iCal on the master device as .ics files in a safe location. This will save all events and reminders as you like them in a way that they can be easily imported again.
Clear iCal of all calendars
With the calendars exported, remove the calendars on your master device so it is now empty. This will allow you to start from scratch when syncing to multiple devices, having cleared any hidden metadata and other items that the calendars may use.
Enable MobileMe and reset sync data for calendars, copying from the computer to MobileMe
With the master calendar empty, re-enable MobileMe on the master device and then go to the "Advanced" section of the MobileMe system preferences and click the "Reset Sync Data..." button. Select "Calendars" and click the right arrow to move data from the Mac to the MobileMe "Cloud."
Sync all devices to remove calendars on them, and ensure all calendar data on all synced devices is removed
With the cloud now reset with blank calendar data, re-enable MobileMe and syncing on all of your devices and synchronize to them. This should result in a blank calendar for all devices. If not, then remove any residual calendars and events that pop up, and sync again so all of your devices are blank.
Restore saved calendars to iCal, and check to make sure they are all intact and working.
Now go to your master device and import all your saved .ics calendar files to iCal. This should restore all of your calendars one-by-one to the master device. At this point, you can either manually sync to MobileMe or wait for the system to automatically sync, and the calendars on all of your synced devices should repopulate.