[ianbooker]

Then you cut out videos you were interested in the first place.

[Alphaeus]

When it asks you why you're not interested, one of the options is "I've already watched this video".

[PopeDotNinja]

I used to do that, but it was a lot of work & I didn't notice an improvement to the search results.

[etripe]

I experienced exactly the same. I suspect that since they're optimising for engagement, already having watched a video is some indication that you'll engage with it again.