if you mean going into the show then I think there were a few years where 2 previous winners turned up and one of those two were victorious...
1972, 1980, 1988, 1989, 1990, 1991 off the top of my head.
there was only one year to my knowledge where 2 previous winners competed but they were both defeated by someone who hadn't yet won the title and that was 1984. (edit... yeah it was 3 not 2)