Let's say you pick flaming. It normally deals +1d6 fire damage. What the "double damage" part is referring to is that if you choose for the base damage of your attack to be fire already, it deals +2d6 instead.
So say I have a longsword. And it does the flaming attribute for a particular enemy. A longsword by itself does 1d8 with the flame attribute it will add 1d6, and with this double damage it will do 2d6?

If that's true then why not do [1d8 + (1d6)x2]?
-"2d6" and "double damage" doesn't seem to be equivalent to me.