Where is fslint (duplicate file finder) for Ubuntu 20.04?

It is not presented in 20.04 LTS repositories because Python 2 deprecated stuff, but you can install it manually:

mkdir -p ~/Downloads/fslint
cd ~/Downloads/fslint
wget http://old-releases.ubuntu.com/ubuntu/pool/universe/p/pygtk/python-gtk2_2.24.0-6_amd64.deb
wget http://old-releases.ubuntu.com/ubuntu/pool/universe/p/pygtk/python-glade2_2.24.0-6_amd64.deb
wget http://old-releases.ubuntu.com/ubuntu/pool/universe/f/fslint/fslint_2.46-1_all.deb

sudo apt-get install ./*.deb

and enjoy using it by running fslint-gui executable.

For 20.10 the solution is a bit longer

mkdir -p ~/Downloads/fslint
cd ~/Downloads/fslint
wget http://archive.ubuntu.com/ubuntu/pool/main/libf/libffi/libffi7_3.3-4_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/p/pygobject-2/python-gobject-2_2.28.6-14ubuntu1_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/p/pycairo/python-cairo_1.16.2-2ubuntu2_amd64.deb

wget http://old-releases.ubuntu.com/ubuntu/pool/universe/p/pygtk/python-gtk2_2.24.0-6_amd64.deb
wget http://old-releases.ubuntu.com/ubuntu/pool/universe/p/pygtk/python-glade2_2.24.0-6_amd64.deb
wget http://old-releases.ubuntu.com/ubuntu/pool/universe/f/fslint/fslint_2.46-1_all.deb

sudo apt-get install ./*.deb

There's also unofficial Snap package created recently, which just packs latest fslint version available in Ubuntu 18.04.

sudo snap install fslint-unofficial

To work with files in external media use snap connect fslint-unofficial:removable-media.

An issue has been opened about it on the official github. Looks like obsolete dependencies are the cause here.

https://github.com/pixelb/fslint/issues/168