Can you defy a hard bargain that was repeated?

Yes, your opponent can use Defy to counter a Hard Bargain, even if you paid the additional cost to Repeat it.

According to RiftJudge FAQ #10472 and FAQ #11066, this interaction works as follows:

1. Repeat is an Additional Cost: You must declare and pay the Repeat cost when you first play Hard Bargain from your hand, before it is finalized onto the chain.
2. Single Entity on the Chain: Even when you pay the Repeat cost, Hard Bargain exists as a single entity on the chain. It does not create two separate spells.
3. Defy Targetability: Defy checks the printed (base) cost of the target spell. Hard Bargain has a printed base cost of [2] Energy and no Power, which makes it a valid target for Defy (which can counter spells with a base cost of 4 or less Energy and 1 or less Power).
4. Full Counter: Because Hard Bargain is a single entity on the chain, Defy counters the entire spell, including the repeated portion. None of the effects occur, and you do not get a refund for any of the costs you paid, including the Repeat cost (per Rule 425.1.c).