If I play Hard Bargain repeated and my opponent pays the 4 energy cost of it, can I play another Hard Bargain on the same spell?

Yes, you can play another Hard Bargain on the same spell, provided you have the resources and the timing is correct.

According to the rules of the chain, players may respond to spells and abilities by playing their own spells or abilities. If you play a Hard Bargain (even with Repeat paid), it is placed on the chain. Your opponent then has the opportunity to pay the energy cost to prevent their spell from being countered. If they pay that cost, the Hard Bargain resolves (and its effect is countered/nullified because the cost was paid).

However, because the original spell your opponent played is still on the chain (it has not yet resolved), you may play another Hard Bargain in response to their payment or simply as a new reaction to the original spell, provided you have the energy to pay for it.

Important Nuances:

• Timing: You must play each Hard Bargain as a reaction to the spell you are trying to counter. If the opponent's spell resolves, it is too late to counter it.
• Repeat: As noted in the FAQ, you cannot add Repeat to a Hard Bargain after it has already been played and placed on the chain. You must decide to pay the Repeat cost at the moment you play that specific Hard Bargain from your hand.
• Cost: Each Hard Bargain you play is a separate instance on the chain. If you play a second Hard Bargain, your opponent will have to pay the cost for that one as well to keep their original spell from being countered.

This is my interpretation based on the standard rules for the chain and playing cards. No official FAQ exists for this specific interaction of chaining multiple Hard Bargains against the same target.