That is a very interesting question Laura. I seem to ponder if on you system - PC or Lap top for personal system files. A term with vendors would be in-house files. Or, do you mean an online service free or otherwise that seeks duplicate content on the internet?
There are some good ones I have perused, yet not ventured for as yet. I ponder if HP's system for duplicate content or that of the Google Webmasters helps. A link is below how Google views duplicate content and possible actions. They offer a means of verifying authorship for articles too incorporating the Google+ account. Not sure if that offers protection from "Scraping" articles, yet it appears to offer your readership a means of verifying if it is your work, thus creating readership loyalty. If your image is no there with your ID then it is not you is what I grasp.
This article offers help with content duplicate and explains a little how sometimes content is generic especially with descriptors. A few changes are suggested will increase a higher range in Google index by page and article placement.
I can't remember if I can place a link or not
I do not know if I answered what you sought or not. For in-house I have only discovered file search and categorizing programs for a nominal fee. Nothing as yet that will search in-house duplicate document content.