Heard about this from a colleague yesterday (http://www.redhillconsulting.com.au/products/simian/index.html.)
Simian (Similarity Analyser) identifies duplication in Java, C#, C, CPP, COBOL, JSP, HTML source code and even plain text files.
I can see where this would be useful on and large-scale project where it is difficult for any one developer to keep track of what has been done. This would quickly identify areas for refactoring etc.
Simian can be easily integrated with your build process as it provides plugins for Ant and Maven