Margaret: I am not a dog.
Margaret: No, I not!
Duncan (growly voice): Yes!
...and it looped around a few times. Sure, it's not their cutest moment, but on the plus side they are both arguing mostly in words these days instead of inarticulate screams and shouts.
Yesterday Laura was noting that Duncan said "yes" way more than any other two year old she knew, and I pointed out that was probably because Margaret had "no" covered.