Interesting story. I want to touch on something that stood out to me in the story first, and then I will give my opinion on the dual agency approach.
When you got the "Offer accepted," what should have happened was that the offer should have been signed by both parties and ratified. Now, I am in Virginia, and I will not pretend I know exactly how RE transactions are done in NY. But here in VA, an offer is only accepted once it is ratified, i.e. signed by both parties, holding both parties to the terms of the deal. It sounds like you got a word-of-mouth acceptance, and your agent should have pushed hard to get the signed contract completed so that the sellers would be in breach of contract by accepting another offer, and you would have grounds for a lawsuit if they did. To me, this was a big blunder on your agent's part.
Now, Dual Agency is a tricky business. My perspective on this as a buyer and investor is that if you are savvy enough to look out for yourself, then it can be a good way to get on the inside of the deal. Technically (once again in VA), if I represent both parties to a transaction with dual agency, I really no longer represent either party. The agent is only an intermediary between the buyer and seller at that point. Now, plenty of agents out there will see this as a way to double their commission, and they will push to make the deal happen, which is not ethical or legal.
As an agent, I don't like dual agency, and I would rather get a referral fee by sending the buyer to another agent so that I can do my job of representing the seller, which is what they are paying for. I have seen that usually, in a dual agency situation, one party feels like they lost out somehow and it opens up the way too many opportunities for me as the agent to get sued.