Luke
Apart from the tools mentioned above, I use copyscape premium to identify duplicate text (in the body of the page), I also find these tools very useful:
Xenu Linksleuth: very good for finding duplicate tags in your page's headers (title, description), and for many other tasks that require crawling your site. And the tool is free!
Screaming Frog: Another web crawler and very good tool for finding duplicate tags. It is a paid tool (about 77 GBP per year) but has a couple of features that Xenu does not have.
Cheers
Jorge