Does the Kineticist Fire impulse Junction stacks?

When you're going single gate on fire Kineticist, the two action blast gains the impulse Junction and the damage dice goes up by one step (d8)

When you're level 5 you can expand the gate and get the impulse junction. So passively the damage is d8, but if you're doing the two actions version, you can apply the junction again and increase it to d10??

Am I tripping here? I think that it's a unique feat that you can't use it twice, but a player started arguing and now I'm really confused