if i target a deflect unit and repeat with bellows breath. how much deflect do i pay? once or twice? if i target the same unit with both instances of falling star, do i pay deflect once or twice?

Twice, in both cases. Deflect is charged per time your spell chooses the unit, not once per spell.

Deflect reads "cost [Deflect Value] more to play as an additional cost for each time they choose me" (Rule 809.1.c). Repeat makes you make fresh choices for the second execution during finalization (Rule 820.2), so choosing the same Deflect unit again on the repeat is a second choose → second Deflect payment. Same for Falling Star: its two separate "Deal 3 to a unit" instructions each choose a target, so pointing both at the same Deflect unit costs you Deflect twice (that's the confirmed FAQ #5107 / #3183 answer).

So Bellows Breath with Repeat hitting one Deflect 1 unit = 1 Energy + 1 Mind (base), + 1 Energy + 1 Mind (Repeat), + 2 Power of any domain for Deflect. Falling Star on a Deflect 1 unit with both instances = 2 extra Power. If you can't pay a Deflect charge, you can't make that choice.

Heads up: a couple of older FAQ entries describe Deflect as a one-time "binary" cost per spell, which conflicts with the current rule text above — the per-choosing reading is the one that matches Rule 809.1.c and the majority of rulings.