Double doesn't just mean doubling the ingredients. It's actually a much more loose term for a "bigger" beer - higher ABV, mainly, but also balanced with a bit more malt because there's probably more hops in there as well.
Try also searching for Imperial IPA recipes, they're the same thing (unless I'm misinformed). Or maybe find a specific one you like and try to find a recipe to match it. Yooper has a great Stone Ruination recipe, and there are some good Pliny the Elder clones out there as well. I'd also suggest the Stone Escondidian black IPA recipe that's been going strong on these forums as well, if you're into that sort of thing.