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.