If you have
AnyDVD u can use IMGBurn in read mode to make a direct 1:1 ISO from the original. Then just use IMGBurn to burn the ISO. I actually havent tried it yet as I havent picked up more DL media yet, but I think it is gonna be an awesome way to backup DLs.
IMGburn in read mode reads each sector and probably would give the best 1:1 backup possible.