To everyone experiencing this, we recommend that they first check whether or not the issue comes up only in the web player. If that's the case, we have an Ongoing Issue about this here. Make sure to +VOTE and subscribe to it, if you haven't already. We'll inform on any new developments there, as soon as there are any.
If you're experiencing this on the desktop and mobile app versions, ti's possible that these are re-uploads of the same track. As @Xenia mentioned in her post, liking a song on the younger album will also save that song in the older version, but not the other way around. If you'd like this feature changed, you can post an idea about it. You can also support the idea about consolidating duplicates here.
Hope you find this useful. Keep us posted if you have any questions.
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“Music is a language that doesn’t speak in particular words. It speaks in emotions, and if it’s in the bones, it’s in the bones.” - Keith Richards