Once a playlist/album has finished, the Spotify radio often recommends a song that has already been played in that particular playlist/album. This means getting repeated songs in one listening session. Therefore, the algorithm should ensure this does not happen (by excluding recently played songs from the suggested songs played after a playlist/album has finished). eg. shuffle a playlist that contains song x, the playlist finishes and spotify radio starts, song x is selected and played by Spotify.
(I think this may occur because of alternate versions of a song, eg. if it's on more than one album or compilation.)