Here's a quick one if your filesystem supports hard links:

    #!/bin/ksh

    typeset -A hashes

    find . -type f  -exec shasum -a 512 {} \; |
        while read hash filename; do
            if [[ -z ${hashes[$hash]} ]]; then
                hashes[$hash]="$filename"
            else
                echo ln -f "'${hashes[$hash]}'"  "'$filename'"
            fi
        done

Pipe to /bin/sh if you're happy with the results.