I found this video on Youtube.com when searching for interviews with Butch Vig. The collections of videos are from a 30 minute documentary about the production of Nirvana's Nevermind record. In this particular video Butch goes over the vocals and how doubling them helped fill the chorus out more.
Butch Vig "IN BLOOM" Video Link - http://www.youtube.com/watch?v=VOYvTXUqlrI&feature=relmfu
Check it out!!
Thanks,
Brian