Duplicate file detecting app

Any recommendations? I've been using a mac since 1991 (mac plus) and have an awful lot of files and duplicated files over different volumes. Is there a recommendation for cheap or freeware programs that could identify my duplicates for me? I've spotted one in the app store (duplicate detective) but wondering what other alternatives there are. Any recs?

Any ideas anyone?

Thanks. There's a lot to choose from, does anyone have recommendations? Are some better than others, are there any to avoid? etc. Is anyone here using one of them and happy with it.