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.
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:
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.