Double and Imperial are pretty much interchangeable. It sort of started with the IPA's and when you make an Imperial IPA it is spelled like this, IIPA. So it is a doubleI PA. It also comes from the Belgian's where they have the dubbel, trippel and quad which just mean that you have more alcohol.
On deck - AAA x2, Irish red
Primary - Baltic porter, IIPA, Mild, ESB, Milk stout, Scottish 80
Secondary - American Barleywine
Bottled - Gingerbread ale, IPA, Brown porter, RIS, Better than dead x2, APA x2, Kolsch