I realise that this is an idea which Spotify has already dismissed (see 'See in which of my Playlists a Song is in'). However, that feed was created in 2012, to be given a 'Not Right Now' status, and then a 'Case Closed' status in 2017.
Nearly a year and a half on from dismissal, I don't see why the conversation of its consideration for implementation cannot be brought up again - times have changed, even in a maybe-considered "short" period of time. The aforementioned feed had 3,816 votes, so it was clearly a popular idea back then, and I believe one still justifiable in implementing today!
The idea is that you would be able to see which of your playlists a song is already in.
As someone who regularly uses Spotify, and creates many many playlists, some of which can be similar, I often get lost with which of the playlists a song may already be in. The service that Spotify currently provides to combat having duplicates of a song in a playlist is a good one, which I do not wish to knock at all. It is where a notification pops up telling you that it is already on that playlist, giving you the option to add anyway and duplicate it, or to remove it and skip any duplication. Though this service is good, going through the process of this for every playlist I'm considering adding a song to (which is sometimes numerous) can be time-consuming.
Instead, I am suggesting that Spotify implement a feature which would allow you to see which playlists the song is already in, and which it is not, to make the process of playlist-creating a lot more fun, easier and smoother.
For instance, a list of playlists that the song is already on could pop up when the song is hovered over/right-clicked/held down (depending on what is appropriate and the device being used), or, when given a list of your playlists (in the process of adding the song to one of them), a star (or an alternative symbol, like a tick or asterisks, etc) could appear next to the playlists that the song is already on - like how a green download symbol appears next to the playlists and songs that are already downloaded on your device. Alternatively, a different mode of making it known to the user could be used - others might have better-suited ideas.
Spotify has already implemented many features which highlight various things for you, that you rarely appropriately appreciate but are definitely very helpful, like the abovementioned download symbol, or how the playlist/song being currently played is written in green. Such ideas are BRILLIANT, and should be given just praise!
What do you think of this idea? I hope you agree that its status should at least be reconsidered (and hopefully implemented)!