A browser extension that uses YouTube's watch history to hide recommendations for videos you've already seen.
By clicking SeenThat's icon in the Menu Bar, you can access the menu and customize what is hidden.
Set a watch-percentage for which any video recommendation will be hidden if you have watched the video beyound that amount.
You can additionally hide other recommended content such as user- and auto-generated playlists.
These settings can be toggled for various YouTube pages including the Watch page, Home, Subscriptions, Channels and Search.
Download and install for Chrome or any chromium browser:
https://chrome.google.com/webstore/detail/seen-that-for-youtube/nlhibcelmcakfgbdmeinlojopoadhnfe