There is a bunch of 'duplicate finder' apps for Windows (so I assume for every OS under the sun). It'll take you 10mins the first time, and 1 min every other time.

I run it every few months (or whenever I remember to do so). I don't bother for word/excel/pdf, even if there are some 20MB PDF files, a 5min 1080 shaky vid from a concert you went 5 years ago is a far worthier target for your time.

Documents don’t take up much space. Videos absolutely do though, so you’ll get a pretty good return on investment in going through those.

for me, i just search for the biggest files in my devices, then dedupe the ones with same file-size. Then I take a look on thumbnails of the big files and dedupe in case the thumbnails are the same (^but the file-size is not). Then I sort all the left files on size and look out for general data of same file-size and then decide whether to delete without breaking things or just leave them.

but, I'm thinking putting a NAS in my home and do all the neccessary backup onto that.