Yup, your assumption was correct. Natty can't be useful to detect duplicate answers. Natty looks at just the post and the question to judge whether it's NAA or not. (We have
another bot on SO to do something similar to that). A "fp" reply to such reports by Natty is the correct one (and yep, they shouldn't be reported). Duplicate answers are flaggable on SO as well, but we use the custom moderator flag for those.