Does falling star deal 7 or 8 damage to a unit on void gaite

8 damage — each of Falling Star's two instances gets Void Gate's +1, so it's 4 + 4.

Void Gate reads "each instance of damage the spell deals to a unit here is increased by 1," and Falling Star deals two separate instances of 3 (Rule 715.2: with multiple targets, Bonus Damage applies to each target individually and separately). So targeting the same unit twice at Void Gate = 8, not 7. (If that unit is moved off Void Gate before the spell resolves, both instances lose the bonus and it's only 6 — FAQ 7604. Stacking Annie, Fiery on top would make it 10.)