Please add an option to add things to a playlist (if you are in a playlist marking things)

Edit: I mean it would be great to select videos in one playlist and then be able to add them to a different one

Great for what it does. Missing a couple features I would like
1) Shift select/deselect to toggle a range. When on the liked videos playlist, I usually need to select a range of related recently watch videos and having to click each check is very time consuming.
2) Add to playlist. I use this for batch categorizing my liked songs. You can still do this by using add to queue button, and then youtube's own save queue to playlist feature.
3) De-duplication feature for playlist/queue, for anyone stupid enough to push save to playlist or add to queue twice. Youtube used to have a builtin de-dup button, but they got rid of it. Not really a feature I would expect from this, but one that wouldn't require much code change to do.