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Is anything else on your Mac syncing with the iCal to-do database?

Unfortunately, the to-do database doesn't have a way to uniquely identify items - you can't rely on the name of the action because it might be modified, or you may have more than one action with the same name.

OmniFocus (and we suspect most other apps that sync to-dos) works around this problem by adding a link to the "URL" field of each to-do. This works to uniquely identify a to-do when a single product is syncing with iCal, but since each can only have one URL attached, if two products both try to sync with iCal, they stomp all over each other. The other product's links are going to look like brand-new actions to OmniFocus, and vice versa.

Another customer had duplicates start showing up in iCal, and we traced the problem back to this interaction.