Free app to detect & delete duplicate files from my Mac
Beware : once you change the content of a torrent, you're not able to seed it anymore.

Open a finder window, search for all files named .dat then just press cmd+a (select all) then cmd+backspace (delete) then cmd+backspace+maj (empty the bin)

You can also use gemini to find duplicates then delete them with Finder, or if you're familiar with bash :

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