simhash

generate similarity hashes to find nearly duplicate files
  http://wiki.cs.pdx.edu/forge/simhash.html
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One of the questions that it's nice to be able to answer about a pair of files is the degree of similarity between them. This command-line tool is useful for estimating the "degree of similarity" between a pair of nominally sequential files such as textfiles. The tool uses Manassas's "shingleprinting" technique.