How does Defy work on a repeated spell? For example, Bellow's Breath, repeated. Do I Defy only the one rep or both entirely?

The FAQ already in the context covers this thoroughly. Here's the ruling:

You Defy the entire spell — both the initial effect and the repeat — in one go.

Repeat doesn't create a second spell on the chain. When you pay the Repeat cost, you're modifying the single spell to execute its instructions twice upon resolution. It's still one spell object. If Defy counters that spell, the whole thing is countered — both the initial execution and the repeated one.

Key points:

• Defy checks the printed (base) cost — Bellows Breath costs 1E / 1P. Paying the Repeat cost doesn't change that, so Defy can target it regardless.
• Repeat is an additional cost paid during casting, before the spell is finalized on the chain. You can't wait to see if your opponent Defies before deciding to Repeat (Rule 746.1.d; FAQ #256, #3189).
• Countering doesn't refund costs — the 1C you paid for Repeat is lost along with the base cost (Rule 412.1.c).

So if you Defy a Repeated Bellows Breath, the spell is removed from the chain and placed in the trash. No damage is dealt at all — neither instance resolves.