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Is it possible that you are tapping enough times to check it off (which creates the next one), uncheck it, and check it off again? I see that on occasion, and it isn't as readily noticed because the iPhone doesn't immediately show the newly created action even if you are showing remaining actions.

I also occasionally will check something off in both places before syncing, which will get you an extra if it is a repeating action (because both clients have created the repeat).