Roon really isn’t the tool for this, or any other library management for that matter.
In the past I have used this tool for finding file duplicates (for jpgs, not audio) but it should work just as well. Free to try too. Other tools are available but I have had no experience. Any tools are going to take quite a while on such a large number of files.
https://www.mindgems.com/products/Fast-Duplicate-File-Finder/Fast-Duplicate-File-Finder-About.htm
One thing to bear in mind, if you or your son need lower quality duplicates for good reasons (eg an mp3 version to play in the car/phone etc) then it is good to know that ahead of deleting them all!
There is also another tool which I have used which can do this but I have always found it to be incredibly slow, even on a small library (it fingerprints the audio), but it may have improved by now as I haven’t used a recent version:
http://www.jthink.net/songkong/
Note that a discount may be available for Roon users at times (see thread…). Again, you can try before you buy and the author has a Roon account here @paultaylor.