This doesn't sound like a "dual agency" issue... rather a "multiple offer" situation.
The listing agent's clients had an advantage over you, in that they knew there was another offer (and no incidentally... how much that offer was for, which is unethical, if not illegal).
You should have an opportunity to amend your offer, knowing now (as you do) that there is another offer on the table. And make sure that you make it your "highest and best" offer.. knowing that you're going to be competing with another offer.
You could also approach the listing agent's managing broker, and explain that you believe that the listing agent used her "inside information" to guide her buyers offer.. and you want to be assured that will not happen again this time... so you'd like to deliver "sealed bids" to the managing broker... this way.. YOU send a bid in to the broker, and the agent must do the same... the agent in this scenario will not KNOW what your new offer is, so they'll have to play fairly.