I guess Spotify's algorithm "Discover weekly" works by looking at the songs added in my "Liked Songs" instance and searching for similar songs, which means in order for the algorithm to be as efficient and accurate as it can be I force myself to be extremely selective. The problem is the music I like to listen from time to time which I avoid adding to "Liked Songs" so it doesn't influence the algorithm.
I am unable to keep track of which songs are already added to a playlist and which playlist.
There is a "Heart" tag to notify if a song is in our "Liked Songs" instance. Why not come up with another icon easily visible? if you own the song in any playlist this icon will be enabled, right or left click on this icon lets you know in which playlists you can find this song (maybe remove it). Add the option to skip songs already found in playlists I own so I can spend more time looking for new music and less listening to the same.
Spotify is the app where I spend most of my time. 80% of my time in Spotify goes in hunting for music I love it but is just annoying not to know if the song is already in a playlist I own or not. Also, I try not to put the same song in 2 different playlists but some times it happens, you can see which songs are in a playlist why not let the user also know if a song can be found in many and which playlists? It is an important tool for organizing my playlists and saves me time in not listening to music I already have in a playlist so I can listen more to new music I wish to add.