Content recompressed with a different file format at a different compression level will almost certainly not match, and many pirate sites use encrypted RARs which will all differ from each other almost entirely.
That's why I think the "Megaupload took down only one link rather than all the infringing content" aspect of this indictment is so scary, especially for a site that does perform introspection into its hosted files - it's easy for a judge to go from "you can send only unique differences" to "you can locate every instance of this copyrighted content and remove it or else."
That's why I think the "Megaupload took down only one link rather than all the infringing content" aspect of this indictment is so scary, especially for a site that does perform introspection into its hosted files - it's easy for a judge to go from "you can send only unique differences" to "you can locate every instance of this copyrighted content and remove it or else."