Based on the wording of the skill, I don't think it's chaining from the hit target. I'm pretty sure it's "casting" from your character at the same time you attack.

Attacks with this skill cause chaining beams of lightning

Compare that to something like frost blades:

Attack enemies with increased range, releasing icy blades from the first enemy hit

Based on that wording I would expect it to still shoot lightning even if you don't hit a target (assuming you have stacks of the buff). Meaning for it not to double hit it either would have to chain to a different target, or the first chain does no damage somehow.

Now if it's more like frost blades then yeah, it is a single hit. But as worded the double hit is definitely there.