I then reimported that secondary catalog into the primary one, and it recognized the renamed files as duplicates, despite them not having the same name anymore. In the secondary catalog, I renamed the files.

Now, I ran another test: I exported some photos out of a catalog 'as catalog', 'with negative files'. Added Thai and Serbian to installer language on Windows. Fixed missing mnemonics in the system-tray Menu - Feeds with lowercase encoding where not recognized as supported encoding in most cases.

So although they seemed to me to have identical filenames, they don't. When parsing Atom Feeds, RSSOwl was not handling CDATA-Constructs which where part of a HTML/XHTML content correctly. Those that have taken the trip through Apple Photos and out again have lost their leading spaces. To try and understand a bit better what is going on, I took a subset of those photos (2 out of the 10 or so years) and put them through photosweeper to see how many duplicates it recognized.ДЄnd this is where I think I figured out the problem: for some reason, many (most!) of my photos in my master catalog have filenames that start with a space or two. So, I tried what you suggested, and unfortunately I still get the same result, meaning that when I look to import the secondary catalog into the master catalog, only a tiny portion of the photos are recognized as duplicates. Apology appreciated, glad it wasn't intentional!

Rssowl not recognizing duplicates