This plugin is not working as described. The code is very short and simple. It basically strips whitespace from the content and creates a hash of the first 1024 characters.
But what if a single character was changed in the original post? Then it's considered a new post.
I believe it should also use strip_tags() or replace the preg_replace regex to only allow [a-z] for the hash and maybe reduce to less than 1024.
Even then, still face the question, what if original source post was updated to fix a typo or bold a word?
I think I will try to modify this, and just compare either content url or post title. Hardly ever do you same same title with different content.