Finding image duplicates in collections

Digikam

The best tool by far is Digikam, it is very convenient to quickly find duplicates and erase them. You should usually select a similarity factor of 65-70%.
Finding duplicates only work if fingerprints have been generated first. This can be done using Tools -> Maintenance -> Fingerprints.
The main default of Digikam is that it cannot work on folders that are not part of its "collections" (although it is quite easy to add a folder to a collection). Also it seems to create a hidden (starting with a dot) folder in some directories, to implement trashes.
Be careful that an album is not the same as a collection. A collection (or root folder) has several albums and is configured via Settings -> Configure Digikam.

A tool that can work as well is Geeqie (in Portage). However I found out its behavior to be a bit strange and thus Digikam is better (also, it's not possible to view easily found duplicates to confirm them, which is very problematic, and Digikam does that very well).
Choose the folder where you want to search for duplicates (you can select a top-level folder and scan recursively), then choose Compare by Similarity (or Similarity - High).
Once it gives you a list of found duplicates, you can select them all, choose "View in new window", then click on the new window to slideshow the images.
The most convenient way to remove the duplicates is for me to move all found images to a new place (other folder) then open the folder in Gwenview and remove duplicates manually.
Digikam also has some duplicate searching features (not tried yet).