It would be nice if Spotify can recognize identical songs to avoid duplicates in liked songs.
This is pretty common for songs released first as a single then within an album. If you like the single one, the UI will not show the song as liked when you browse the album. And if you like the song in the album, you'll end up with a duplicated version in your library. This also happens a lot for songs that you listen in Daily Mix or Discover weekly. If it's not the exact same song you have already liked before, the UI is inconsistent and you can save a duplicate.
Apparently, there is already an internal merge mechanism as the play counter is not split between the different versions of the songs. Maybe this can also be used to consolidate the UI.