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If you happen to save a song that is already in your playlist, the web interface will allow you to add a duplicate song. The iOS app will reply that "the song is already in playlist, add duplicate?".
Would be great if that same feature existed on the web version.
When you save a song to a playlist, do not show a playlist that already has the song. My playlists can be very large, and I don't always know if a song is already on it.
@LastDJ I have moved your post to the Feature Requests space under this thread to keep the forums organized. It looks like there are others who are looking for the same feature to get added to the service.