Free app to detect & delete duplicate files from my Mac

I am just wondering is there any way to find duplicate files on my Mac?

For example: when I download something from bittorrent, some unnecessary files also download along with the main file with .dat extension. I want to delete all of these .dat files at one time. Currently I have a lot of these files on my computer & they are stored in many different directories. It would be a lot of work to delete them one by one.

Thus, are there any free apps to do that sort of job or some way I can find & delete them?

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

You might want to take a look at Gemini. Its $10US, but it is a very powerful utility. I've used it with iTunes music folders and numerous folders of camera card dumps. A colleague of mine uses Gemini to prune through 15 years of digital files, many of which were duped as backups when technologies changed at my school. (Disclosure: I have no financial interest in the Gemini app.)