Sometimes this downloads a wrong song. Example: I'm trying to get this song:

https://open.spotify.com/track/6Fex0cdE0X35HSB1PgwnnR

what it downloads instead is this song:

https://open.spotify.com/track/1VIEstKwksrGK0Gb4EOzxl

Same title, even similar artist name. Not the same song at all.

This would appear to me to be a caching issue... You probably try to get the audio from Spotify only once, and then cache it. And I assume you're using a very dumb cache name.

I've also had a case where it downloaded a different version of the same song.

Guess what: the URL already provides a proper cache name, in that last part (the hash) of the URL.

p.s. Perhaps another remedy could be to have the user login to Spotify via OAuth, if possible, and use that account to download the song?