Awhile back I started using a video's 'like' button simply as a mechanism by which I could tell if I had watched something before since I did get repeat recommendations. Then, if I clicked a recommended video again later, I'd know I had already seen it. Over time, as far as I could tell, YT stopped recommending videos I'd seen before as there were fewer and fewer videos I would check out that I had already seen.

May also be attributed to something else, or I'm getting old, but it seems to have been a fortunate outcome.