2. Double Tap? Double tap says you use double the ammo. Biohacks are not ammo. Using the Double Tap feat would have you doing a single attack, that may apply a biohack if you choose, but it only uses a single biohack usage. . . because it is a single usage of that ability. The doubled needle ammo usage is what pays for the DT bonuses. The same would apply with stuff like Overcharge Weapon or any other ability that effects an attack. Double Tap enhances an attack, it doesn't make it two attacks.
I agree it uses up 2 usages worth of your battery.
I yield that mechanically, it doesn't matter what you load into it, it just uses up two uses of whatever ammo it's using and does it's thing.
However In the case of Biohacks (or really any situation where it matters what ammo you use) doubletap consuming twice the ammo can make some questions happen.
This is going off topic a bit now, but lets say you loaded up a Needler Pistol with 6 doses Blue Whinnis poison and the used double tap with it. Are you completely wasting one of your doses of Blue Whinnis? Or do they get exposed twice upon a hit? Would it make more sense to load every other needle in the gun with the poison if you're going to Double Tap?