One day some time ago I discovered that DO had a Find Duplicate Files function. Wonderful - I could get rid of another application that did that. It never ceases to impress me how useful DO is.
When there are a lot of duplicate sets though, a useful thing that the stand-alone app could do was to set a path (or list of paths) to either keep, or delete, preferentially.
For instance, I could set it to keep a file if found in the folder c:/pics/cars/ferrari and mark for deletion those duplicates found elsewhere (and additional duplicates in the given folder). Or, inverse it and mark files in the given folder for deletion first.
In either case, only one file in a duplicate set would be kept unless I manually intervened. This is mostly what I want; leave one of each file where it should be, which might be all in one main folder, or in many different folders (different files, that is) where I've sorted them to and removed from the given one.
There was also a checkbox to include subfolders below the given path as well, so a file found anywhere from the path downwards would be 'preferred' for whichever.
This occurred to me today when I found about 200 duplicate sets, and of course all the ones marked for deletion were in the one folder I wanted to keep them in so I had to manually inverse every file tick.
Note that I did discover, after doing the above, that I could reverse the sort order in the duplicate files display and click 'Select' again. That might do the trick sometimes, or might not. Having the above would be more flexible.