In over 40 years of messing with computers, I would not recommend implementing a duplicate scan function in CCleaner. This function is best left to a separate utility for a number of reasons.
1- Compared to systems just even of 10 years ago, for example, today's computers are more focused on the user experience. A lot of the techno jargon is being glossed over and hidden. This opens up computers to youngsters and people that just want to get some serious work done. Messing with the operating system and arbitrarily removing files can break a system subtly or majorly. We all like 1-click utilities, but a 1-click duplicate remover is just asking for problems.
2- A user will be less inclined to (when running CCleaner) arbitrarily go and delete duplicates which would break the o/s; if CCleaner doesn't take on the task of duplicate removal.
3- Some applications (and games) use identical files between major versions. I'm a classic gaming enthusiast and here are some examples!
* I have Duke Nukem 3D from the 1990's and I like the nostalgia of playing this old game. I also like some of the mod packages. One of them is eduke32. It basically consists of updated textures and a new rendering engine. With both versions I can enjoy the old and the new. If I ran a dupe finder it reports that some maps and graphics files are shared between each version. Bam! One of those will break! The same thing applies to DosBox and the various versions of Doom.
* I have multiple installs of X-Plane flight simulator and Orbiter spaceflight simulator. Running a duplicate finder would immediately kill these, no doubt.
* In emulating classic gaming systems like the Atari 2600, one often builds up a rom collection. There are hundreds upon hundreds of tiny 4k game rom files. It is common to use the directory tree structure and the filenames themselves in WindowsExplorer to help categorize the games by mfg or genre or other criteria. Since the files are so tiny they take up like almost zero space. Running a dupe finder would mess up the selection menu's appearance big time!
* A non-gaming example; this would be Photoshop and graphics editing in general. I tend to organize projects by directory, a solid strategy. Nice and neat. I think you'll agree. Sometimes there are duplicate images and elements and the like spread between these projects. Running a duplicate file finder here could mess up a project. An offshoot of this is your photo album. Perhaps you WANT a picture that is of two subjects to be contained in multiple collections or sub-albums.
4- All duplicate finders are not intelligent. This is fact. They can in no way know what you want to keep and what is important or not. These dupe finders are tools to be used in the process of doing something. And that something is removing file. You don't want to let it run unattended, and you always always always want to approve the recommendations and file list prior to deleting.
5- Modern O/S'es - this is a biggie, a huge no-no! There could be several hundred megs of duplicate dll's and executables and other libraries buried in your Windows directory. I do not (and neither do you) know what each file does. Perhaps it's a leftover from a system update, or a backup copy of a low-level system file ready to be restored by Windows in case of emergency. Maybe two separate applications put the same file, albeit with different names but same content, in the same (or different) Windows sub-folder. Who knows? Some of these duplicates could be for system maintenance too. Are you going to research each and every file? Are you going to retest your important applications after removal of each file?
6- With a properly configured system, there would be little to no advantage in removing o/s dupe files anyways, other than to get more disk space. The overall speed of the system isn't going to be affected a whole lot. This is really an offshoot of the above item.
7- Today, in the era of big disks, there's no need to aggressively trim your O/S and program/application folders. Even with SSD's. This isn't the 1980's where HDD's were 10 and 20 megs in size. You get everything working right at the outset and make a note of how much space is used. Write it off and call it a night. If you're hemming & hawwing over a 200Kbyte O/S component then I don't know what to tell you except that you need help.
8- Keeping the two flavors of utilities (dupe finder & garbage remover) separate will also keep your frame of mind separate. You will less likely make mistakes when you concentrate on one task at a time. The race to remove everything is much less when the tasks are done one at a time.
Having said all that, I do believe there is a place for duplicate removers.
1- They're perfect for finding things that you may have made 2 copies of, while in the process of re-organizing your personal data. This includes music files and photos and anything else you hoard.
2- If you do a lot of downloading and aren't meticulous about where you put things, you may find you have an equivalent download.tmp and newgame.zip.
3- Excellent for going through old archival material you may have forgotten about. I'm sure we all have duplicate movies and download install files we no longer need. Perhaps you do a lot of documentation work or the like. This is also good for backup management if you do a lot of file syncing
4- Freeing up disk space. Yes, that spiffy 3TB data drive can fill up pretty quick! And trimming off 20megs here'n'der can extend out the purchase of a new drive, or perhaps allow you to squeeze in another "dataset", whatever. Just as long as you aren't attacking the o/s and application folders - everything's kosher.
5- If you're downloading document files for archival purposes, a duplicate finder can quickly determine what are new additions and what ain't. Let's say you have a collection of old scanned computer manuals from the 1970's, and you come across a website that seems to have even more of them! So you download what you think you don't have yet, and compare your existing library against what you just acquired. Then you know if you need to add new content or not. This is good practice that works over time.
CCleaner's functionality should remain as it is, and the task of duplicate removal left upon a separate stand-alone utility. I'm sure there are counterpoints to all these points, so let's hear 'em!