From http://www.commandlinefu.com/commands/view/3555/find-duplicate-files-based-on-size-first-then-md5-hash

find -not -empty -type f -printf "%s\n" | sort -rn | uniq -d | xargs -I{} -n1 find -type f -size {}c -print0 | xargs -0 md5sum | sort | uniq -w32 --all-repeated=separate