The recent OS X 10.6.3 update is causing problems for some but not all Spanning Sync users.
After the update is applied, Apple Sync Services erroneously sends Spanning Sync a potentially large number number of records as "added" which are in fact existing records. (Technically, Sync Services is performing a "fast" sync, where it should be performing a "refresh" sync.)
In our own testing, Spanning Sync's duplicate detection and prevention mechanisms prevented any duplicates from being created, but two of our users have reported finding duplicate events in their calendars after applying the 10.6.3 update.
We recommend backing up your iCal calendars (File > Export > iCal archive...) before upgrading to 10.6.3. If duplicates are created after upgrading, you can just restore your calendars from that backup and replace your Google calendar data using Spanning Sync (Advanced > Replace Data on Google > Calendars).
If you already have duplicate events in your calendars, please contact email@example.com and we'll help you quickly remove them.