Usually double, or imperial, beers do not actually have twice the grain bill. Like the bock to doppelbock relationship, it's a stronger beer, but usually not doubled. Doppelbocks may have anywhere from 20% more grain to a full doubling (though this would be rare).
For an IPA, where a typical gravity might be 1.060, I would probably aim for increasing ingrediants by 50% to get you up around 1.090. That would make a great IIPA.
Currently On Draft: Bamberger Rauch Dunkel, Belgian Blond, Pilsener Urquell clone, Smoked Porter
Bottled: Concord Pyment, Mi'Apa Sparkling Mead, Chimay Blue, Old Simcoe American Barleywine, Old Cantankerous
Fermenting and Conditioning: Pseudo-Decoction Munich Dunkel, Left Hook Bitter
Recently Kicked Kegs: Fresh Hop Pale Ale, Citra Rye IPA
On Deck: Old Rasputin, Northstar IPA, Ur-bock Dunkel