It's knows not be true random "It doesn’t like songs that are totally different bumping up against one another" and users wouldn't like if the same song is played twice in a row regardless how logical that would be with rand().
https://lifehacker.com/the-reason-spotify-shuffles-aren-t-re...
https://medium.com/immensity/how-spotifys-shuffle-algorithm-...
Even since the iPod, it was found that people don't really want real randomness when they shuffle their music.
Here is related Spotify Engineering blog post from 2014:
>How to shuffle songs?
https://engineering.atspotify.com/2014/02/how-to-shuffle-son...