The duty is opt-in. Only you can assign yourself that duty. No one else.

In the extreme, I think it would be doable to argue that we don't even care if the output you produced is in fact spendable (though it would be nice because everyone tracks it.) The only things we care about is that you're not counterfeiting your input amounts, and that miners aren't rewarding themselves more than the agreed upon block subsidy + fees. Everything else we really care only about when we are receiving a transaction.