n3k4

To help your WordPress installation be more optimized for search engines, this plugin will include a ‘noindex’ meta tag in the header of any duplicate posts. 
Robots will not index any posts with matching title tags, which is a known indicator of negative SEO. This is very useful for automated posting and working with a large amount of posts.

Noindex Duplicate Posts

This plugin will automatically add a 'Noindex' meta tag to posts that are a duplicate determined by the slug and preexisting post names.

Badi Jones

Duplicate Content Cure is an extremely simple plugin that makes WordPress more Search Engine 
friendly (And therefore improving SEO). It does this by preventing search engine spiders/bots 
from accessing pages like Archives, Tags, and Categories, which tend to contain duplicated content 
from elsewhere on your site. This is achieved by adding nofollow, noindex to 
these problem pages.
Duplicate Content Cure doesn’t have a lot of features that other SEO plugins have, but 
it also is lightning fast by comparison.

Duplicate Content Cure

Duplicate content cure is a simple plugin that improves SEO by preventing search engines from indexing pages that contain duplicate content.

Noindex Duplicate Posts vs Duplicate Content Cure