There has always been confusion regarding what a double roll is!    A double roll is ONE ROLL of wallpaper which is usually 20.5" wide by 11 yards long.  Many years ago you could order one single roll and the manufacturer would cut a double roll with a cutting fee but they no longer cut rolls.  Now you still have the same wording but you cannot get one single roll, only one double roll. 

Again a double roll of wallpaper is actually one bolt that is 20.5" wide and 11 yards long.