So I just loaded up a 13w42b world, and almost immediately, my favourite track, mice on venus, started playing. I was enjoying the lovely piano music, when something got my attention. I could have sworn I heard something else behind it. I thought it sounded a bit like the beginning of the song Clark.
As Mice on venus continued, the music behind it got louder. I knew it WAS Clark. Two songs were playing at the same time! I thought it was a horrible earsore, but I continued playing anyway, listening to the two songs playing at the same time.
Then DRY HANDS started. I realised this was too much for me, so I slammed the speaker volume down to zero. Three songs playing at once! It was really nasty and I HOPE this bug gets fixed soon.

I was expecting to be playing for the usual few minutes then hear a C418 background song start, as normal, then hear another one about 20 or so minutes later.

I heard a C418 background song as soon as I entered the game. After only a minute or so, another one started behind it. Then ANOTHER one came on another minute later.

Steps to Reproduce:
1. Load up a 13w42b world.
2. Go on Creative (that's the mode I was in, dunno about the others).
3. Wait fora while and listen for multiple songs to start playing.