Why is there 2 of the exact same midi in DOOM.WAD

[Telehack]

i was looking thru ultimate doom just to see whats in there and whatnot and i noticed that all the songs have a copied version of them thats the exact same (btw i say midi but they are in mus format) 

[Shepardus]

It's not just the music, a lot of the sounds are like that too. I think everything that's in the shareware got duplicated.