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.

jonkemp

Adds the noindex meta tag to your WordPress Login page.
If your WordPress blog has a public login page, you probably don’t want it to be indexed by search engines. Noindex Login adds the robots “noindex” meta tag to your login page to keep search engines from indexing it. It also works on your registration and lost password pages.

Noindex Login

Adds the noindex meta tag to your WordPress Login page. If your WordPress blog has a public login page, you probably don't want it to be indexed …

Noindex Duplicate Posts vs Noindex Login