Here's a screenshot of a comment thread being duplicated. Notice it says there are 5 comments but more than that are being displayed, and the extras are just duplicates of the first 5. I can't get a screenshot of the thumbnails being duplicated since I have to scroll about 3 screens worth to start seeing the duplicates.

I also tried it in troubleshooting mode after deleting site data for youtube and google. It still happened before logging back in and after logging back in, so it is not caused by add-ons and happens whether or not I am logged in. It happens on all threaded youtube comments (comments that can be expanded to see additional comments made to them) regardless of what video they are under. The duplicate thumbnails are shown on the Recommended page.