Whole-folder answers
Two questions you actually have about folders, not files.
Going through sets one at a time is the wrong tool when an entire folder is a copy of another one. Deduplo answers that directly. Both buttons appear above the results only when there is something to show.
Folder copies
Pairs of folders that share most of their contents. Open the list, pick the side you want gone, and every duplicate in that folder is ticked in one click — the other folder becomes the keeper.
Files in that folder that have no copy in the partner folder are left alone. Marking a folder copy can never leave you without a file.
Redundant folders
The one-directional version, and usually the more useful one:
Is everything in this folder also somewhere else?
That is the question worth asking before deleting an old backup drive or an "Old phone" folder. If the answer is yes, the whole folder can go — and Deduplo will tick exactly the files that have a copy outside it.
A file is only ever ticked when a copy of it survives somewhere else. If every copy of a photo lives inside the folder you are clearing, that photo is skipped.
Related sets are excluded
Burst shots, RAW + JPEG pairs and photos grouped by date or place are related, not duplicates. Neither folder answer counts them — so two folders holding one afternoon's shooting are never mistaken for copies of each other.
Folder breakdown
Result ▸ Review & insights includes a folder breakdown: which folders hold the most duplicates and how much space each could give back. It is the fastest way to find out where a mess actually lives.