I went trough the cumbersome exercise of cleaning up my Pictures library, which consists of more than 20 000 images. Needless to say it's completely impossible without a good tool.
I usually use fslint under Ubuntu, but there are not so many options. Under Linux, one can do quite powerful tools in a one line bash code. However I was not in the mood for that... I remembered that I used a good Windows utility in the past, but I couldn't find it anymore. So I googled a bit, and here is what I found out:
Duplicate File Finder by Funduc Software does its job without any extra feature, and is not free. So let's forget this one. Duplicate Cleaner from "Digital Volcano" on the other hand was a saver, very easy to use, yet powerful and customizable.
Duplicate Image Files
I found out that VisiPics by Guillaume Fouet was quite impressive for a free tool. VisiPics does more than just look for identical files, it goes beyond checksums to look for similar pictures and does it all with a simple user interface. And it works well!