I don't think dedupe seriously combats use in infringement - it just forces pirates to repack/recompress their infringing content into many different forms, which pretty much happens anyway since lots of "torrent warez" groups like to put their names and spam links on everything.

From a legal/rights-holder standpoint that's a double-edged sword - it could be argued that because Dropbox dedupe files, they have a head start on DMCA compliance, but it could also be argued that the technical infrastructure means they should be better at removing infringing content.