This is a feature on the search results page and it has actually been around in previous versions.
Here is the explanation of how it works. I found it on this TechNet blog:
http://blogs.technet.com/b/harikumh/archive/2008/11/14/some-interesting-facts-about-sharepoint-2007-search.aspx
"How is the duplicate document identified when we do a search?
Document similarity for purposes of identifying duplicates is based only on a hash of the content of the document. No File properties (e.g. file name, type, author, create and modify dates) are input to this hash. The MSSDuplicateHashes table in the SSP’s search database holds, for each document, all the 64bit hashes necessary to determine if one document is a near-duplicate of another. This is read while doing a search if duplicate collapsing is enabled."
