Again I'm not sure, but if you want to make sure that you have everything that is currently available, then I guess that you should buy the game twice, yes. As far as I know, there are at least two versions out there, which is the Steam Deluxe Edition and the boxed Special Edition. There could be a third version actually, offered by D2D. I recall reading that D2D offers an additional map pack, but someone will have to confirm this.
Yes, this is all very confusing and can be quite frustrating if you want to have the lot. I think that civilizations and map packs will be offered as DLC at some point, though. So if you want to have something tangible, I'd get the boxed Special Edition.