A common situation:
1) You love Spotify so much,
you use it every single day.
2) You find a good song and you save it to Your Music,
where you already have zillions of songs
3) So you also want this song into your playlists,
which you kinda sort songs by a personal criteria that works for you
3) But this song happens to fit several of your playlists.. and you wonder:
Q1: Is this song already in my playlists?
Q2: If so, in which one/ones?
Here's a simple solution:
1 ) right-click any song
2 ) show ✔ next to the playlists that contain that song

Extra features (1): Add how many times a song is repeated in the playlist:

Extra features (2): Show ♫ if a song is in your playlists:

With this feature you could browse any user playlist or any search result and figure out which songs are in your playlists at once, without checking each of your playlists manually.
Do you like the idea? Give it some kudos! 😃
Thanks,
Albert
Edit: The idea has been reposted here
Updated on 2017-08-30
Hey Folks!
Thanks to everyone for their contribution to this thread. We noticed this idea has been marked as Not Right Now for a while, so we thought we’d update everyone on its current status.
After serious consideration, we’ve decided not to introduce the ability to see which playlists a song is in. As a result, we’ve marked this idea as Case Closed. Don’t worry! Should you try to add a song twice to the same playlist, you’ll receive the following warning message:
The team behind Search are also working on new and innovative ways to improve your Spotify experience. Although we’ve decided not to implement the feature as described, we appreciate all the feedback the thread generated, and rest assured we’ll use it to inform our decisions going forward.