It counts as duplicated content.

A 301 is a redirect. It sends to the client the instruction to reload the page in the given Address.

A rewrite happens only on the server side. So for the client it's like two different sites with the same content, and this hurts your ranking.

Now... Ok that I didn't explained that on the parent post (didn't thought it was needed), but is it enough reason to a downvote? :P

edit: fixed grammar.

Rewrites in nginx are redirects.

Only a subset of rewrites are redirects. A common usecase for rewrites is to change the request URI to make it match another location block in your config, but without returning 301 or 302 or any other redirect code to the client.