On Spotify, the same song gets to be published multiple times, either as a EP, or album, extended version, radio edit, part of a compilation, and so on.
Let's say a track is released as part of a mix, and i add it to a playlist. Several months later, the same track is released as an standalone track, along with an extended version. I add the extended version to the playlist, not knowing i already have the mix version. So i end up with a duplicate. And this happens lots of times.
I suggest adding a tool to tell me everytime i add a song to a playlist, if there is already a song with a similar title by the same artist in the playlist, and if so, if i wish to remove it.