You don't do one paper for Double Science GCSE. For the linear papers, you do 3 papers each lasting 1 and a half hours. Each paper covers one of the 3 aspects of the science GCSE - Biology, Chemistry and Physics. It usually requires twice the teaching time of a normal GCSE and therefore has two grades. I think GCSE single sciences include a grade for each science. Some people just do one or two of these though. There is additional material for single science, so it takes longer to teach - it takes less time to teach Souble Award than the sciences as three separate diciplines. With DS, your performance on all 3 papers is combined and amalgantes into two overall grades - you get your grade twice basically.
I hope this helps