Can you defy a repeated Bellows Breath?

Yes, you can counter Bellows Breath with Defy, even if the Repeat cost has been paid.

According to the official Riftbound FAQ (ID 8488):

1. Defy targets the base cost: Defy only evaluates the base energy and power cost of the spell on the chain. Because the base cost of Bellows Breath is 1 Energy and 1 Power, it remains a legal target for Defy, regardless of any additional costs (like Repeat) that were paid to play it.
2. Repeat is not a separate spell: Paying the Repeat cost does not create a second spell on the chain; it modifies the existing spell to execute its instructions a second time upon resolution (Rule 746.1.d).
3. Countering stops the entire effect: When a spell is countered, the entire spell—including both its initial effect and any repeated effect—is negated and placed in the trash (Rule 412.1.a).

Timing Note: Since you are playing a reaction to an existing spell on the chain, this is a CLOSED state. Defy will be placed on top of Bellows Breath on the chain. When the chain resolves in LIFO (Last-In, First-Out) order, Defy resolves first, countering Bellows Breath before it can execute its initial or repeated instructions.