Yes, the rewards are the same each time you do any quest. And I'm pretty sure that applies to all quests in GW. The only "exception" is that you only get 1 end-game reward trophy (like Droknar's Key, Book of Secrets etc.) on your first time completing the last mission of a campaign.
Just in case you're curious, I'd like to share some other information just in case.
The 4 main DoA areas (Torc'qua, Stygian, Ravenheart, Foundry) are all connected and all reward you with one chest after killing a boss (so 4 chests in that entire explorable area). The first chest give you 1 gemstone (2 in HM), the next gives 2 gems (4 in HM), then 3 gems (6 in HM), and the last gives 4 gems (8 in HM). Each gemstone type is given by its respective area so:
Torc'qua -> Margonite Gems
Stygian -> Stygian Gems
Ravenheart -> Torment Gems
Foundry -> Titan Gems
So it's quite profitable to try to do all 4 of them in one session. Just make sure to have a Mercantile Summoning Stone because a large amount of enemies will give a large amount of drops.