This poster was much discussed in 2008-09 on MPF and nobody really knew the answer. Jon/Treegate claimed they were fakes but he didn't seem to have any special knowledge.
Most of us, including me, are just "enthusiastic amateurs," and this one really goes beyond that level.
Certainly it is possible to create a DS reprint of this quality but I still have a hard time believing it is sufficiently profitable to make it worthwhile, especially since only a relatively few hard-core collectors really care whether it is "original." (Most buyers just want something to stick on the wall and couldn't care less whether it's double-sided or has fuzzy text. Also, Dark Knight is the kind of super-popular movie that attracts non-collectors to want movie memorabilia of it.)
I've not aware of any DS reprint with a lighter/whiter reverse, although I suppose it would be relatively easy to create that effect through Photoshop.
The wrong sellers - MovieGoods and Bradburied (not including HG) - are selling this, which alone makes it dubious.
HG's source appears to be legit, which tends to legitimize them.
The slightly odd sizing is a negative.
We know that this poster was used in some campaigns. Here's a German A1 which no doubt is legit:
The MovieGoods poster obviously cannot be an "international version" since it has the PG ratings box.
It is possible that the ad agency would know. Their website includes the "Armed Joker" style (although it says "coming soon.)