The case I most often encounter is "oh, you liked x? I'm sure you'll like every remix of x for the next month of your discovery weekly!". Technically not the same song.
Perhaps you've listened to the regular version of a song, but the Discover Playlist has surfaced its "radio edit" or some other very, very similar version?
Something is wrong. Discover Weekly is supposed to have only songs you have never listened to, even once. That's how it works for me.