[Brian]

Sorry for the confusion here, Harringg. The way iCal's to-do database is designed has a few shortcomings. For example, it doesn't have a built-in way to uniquely identify items. (You can't rely on the name of the action for this purpose: it might be modified, or you may have more than one action with the same name.)

It's relatively simple to work around that shortcomings in a way that makes it simple to sync one other application with iCal. OmniFocus (and we suspect most other apps that sync to-dos) uniquely identify each action by adding a link to the "URL" field of each to-do.

This works when a single application is syncing with iCal, but each to-do can only have one URL attached. If two or more products both try to sync with iCal, their modifications to the URL field stomp all over each other.

The other product's links are going to look like brand-new actions to OmniFocus, and vice versa. I suspect that's why the duplicates started showing up when OmniFocus was added to the mix.