Singlemizer is stable and fairly speedy, and finds true duplicates only.
This doesn't mean it's perfect. So that you know what to expect, these are my gripes (not show stoppers, mind you, otherwise this wouldn't rate it four stars):
1) Singlemizer is not exactly easy to use, unless you know answers to "what is the UTI for a jpeg image", for example (???? - that's what I thought). A few more sliders and popup lists would be helpful.
2) Singlemizer is not fully keyboard controllable (setting the delete checkmark on individual files still needs the mouse).
3) Singlemizer compares files only. There's no option to compare contents of folders and delete the folder only if it's exactly the same as another folder. Or, if there is such an option, I have no idea how to use it.
4) For every file, the console logs "file access by '/rsrc' was deprecated in 10.4". I'm thinking scanning would be faster without that warning.
But overall, the price is right and it surely beats some of the other options I've tried, especially when comparing a large number of files (10,000 or more).