The duplicate-finder download trap
Search "duplicate photo finder" and watch where the internet takes you. Download pages with three buttons, only one of which downloads the thing you asked for. Software that scans cheerfully, announces "4,312 duplicates found!", and then reveals that removing them is a premium feature. Installers that arrive with a browser toolbar, a "PC optimiser", and a new default search engine you didn't request. This is one of the grubbier neighbourhoods of the consumer software world, and it's worth understanding why — because the reason explains what to do instead.
why this category, specifically
Three ingredients, and they're a perfect storm. The user is anxious — usually facing a full disk or a chaotic library, motivated to install almost anything. The value is easy to demonstrate and hard to verify: a big number of "found duplicates" feels like proof of work, and nobody checks whether the count is honest. And the software needs deep permissions anyway — reading every photo folder is exactly what a duplicate finder does, so the invasive access raises no eyebrows. Adware could hardly ask for a better disguise.
The paywall-after-scan pattern deserves its own mention because it's so widespread it's practically a genre convention. It works precisely because the scan takes a long time: after twenty minutes of watching a progress bar, "$39.95 to delete" feels less like a shakedown and more like a completed transaction. Sunk cost, sold back to you.
what a duplicate finder actually needs to do
Stripped of the theatre: shrink photos, hash them, compare the hashes, show you groups, delete what you tick. It's a few hundred lines of honest code. There is nothing in that list that requires an installer, an account, a subscription, or a "system optimisation" module. The maths is public and forty years old. The category's business model has no relationship to the work involved — which is the tell.
the escape route
This page was built as the version that couldn't do those things even if it wanted to. It installs nothing (a browser tab has no purchase on your system). It can't scan anything you didn't hand it (the browser's folder permission is granted per-session, to one folder). It can't upload (there's no server behind it — check with airplane mode). It can't hold your deletions hostage (there's nothing to unlock). And it can be read: view-source works on the whole thing, which is a strange sentence to write about a tool that deletes photographs, and precisely the point.
If you'd rather use a desktop tool, plenty of genuinely good open-source ones exist and deserve support. Just be aware that the search results for this category are not sorted by trustworthiness, and that a tool asking to see all your photos should be one you can see all of, too.