Am I understanding this correctly? You knew that your agent was acting as a dual agent and that was not your issue. What troubled you was the fact that your agent did not disclose the fact that the buyers were the same returning buyers from a month ago and the fact that your agent withheld that info has made you question her ethics. Do I have that right?
My first question is.....what would you have done differently if you would have known? You indicate that would have changed quite a bit your response. Are you saying you would have held out for a higher # or different terms because you believe they would have met your terms or $$?
We can't replay the offers and counters now, so you probably won't ever know.
Do you think your agent withheld this because she considered it irrelevant? Do you think she withheld the info because she thought she was helping you be objective about the offer? Do you think she withheld the identity to help the buyer? I am not speculating on the motivation of the agent, only generating inquiry into the variety of potential reasons.
Nonetheless, right now, you the seller client have lost some faith in your agent.; You should pick up the phone and discuss this candidly with her, or her broker.
What do you want right now? Do you want an alternate agent to repersent your interets through the rest of the transaction?
I am not sure why the agent withheld the info. It may have not have been ill motivated.
You, as a seller, need to feel confident from now through the completion of your transaction. Your agent and her broker owe that to you. It would not be unreasonable for you to ask the broker to take over for her, or assign another agent to represent you. Maybe a candid discussion with the agent and her broker can reinstate confidence. Maybe not. But, you are entitled to that, so don't bury it. Make that call to either the agent or the broker.