The only way you should end up with a duplicate is if you complete the same repeating item on two different devices without syncing between. I change or complete different items all the time on my flotilla of devices (2 Macs, 2 iPads, 2 iPods) and the sync code handles it without difficulty, even when I don't have a chance to sync between using different devices. However, when you complete a repeating item, a new one is created, and the sync code doesn't know how to take the two copies that are created when you complete it on two different devices without syncing and combine them into one (if you think about it, this can get pretty hairy, as you might have then changed each one before syncing, or even completed them and created more!).
It's a good practice to sync whenever you start using a new device (hopefully this is obvious — you want to make sure you are using the latest data) and when you are done using a device (makes sure the any changes you made are available for other devices to pick up, and makes database compaction more likely, which helps performance). The program does try to sync a minute after you make a change, but if you've closed the lid on your laptop, or iOS has killed the app after you launched something else, that might not happen, so I tend to tap the sync button out of habit.
Last edited by whpalmer4; 2011-10-27 at 04:24 PM..