Julian Yanover

Check comments for duplicate content using Google AJAX Search API as they are published. This helps to avoid having duplicate content in your posts. It only checks approved comments that have over 50 words. The plugin searches two random strings, both 8 words length. If one of them finds an exact match, it emails you letting you know so you can check it yourself and decide if you want to delete it.

No Duplicate Content in Comments

Check comments for duplicate content using Google AJAX Search API as they are published. Emails you whenever it finds a match so you can check it and  …

ClaytonL

Hate duplicate content? Hate wasting server resources?
Replytocom Controller gives you the option to remove “?replytocom” from comment urls and optionally redirect visitors & bots who try to use it anyway to a destination of your choice.
For those concerned about duplicate content for SEO purposes, the benefits should be obvious.
If you want to speed up your WordPress server, the redirection will let you stop wasting resources serving up the same page over and over again to a crawler bot. Even worse, pages with a question mark in the url aren’t typically cached by caching plugins like WP Super Cache. That means that those bot crawls are eating up way more resources then a normal url would. Now you can send them to a cached copy of that page, a less resource intensive page, or any other url.
Of course, the cost of all this is that visitors can’t have JavaScript turned off if they want to leave replies in threaded comments.
Borrows code from: 
* https://wordpress.org/extend/plugins/all-in-one-seo-pack/ 
* https://wordpress.org/extend/plugins/replytocom-redirector/

Replytocom Controller

Replytocom Controller lets remove "?replytocom" from comment reply urls and redirect bots who try to use it anyway.

No Duplicate Content in Comments vs Replytocom Controller