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.

Patryk

The plugin adds a “noindex” field in the edit window for pages, posts and custom post types (CPT), which, when selected – after saving, adds the “noindex” tag to a given page. The tag blocks the indexing of a given page in search engines, e.g. Google.

PRyC WP: Add noindex tag

The plugin adds a "noindex" field in the edit window for pages, posts and custom post types (CPT), which, when selected - after saving, adds …

Noindex Duplicate Posts vs PRyC WP: Add noindex tag