As you have explained the situation it sounds like the seller posted the property as "Back ups needed" once your contract was in place. If this is the case it is a normal real estate procedure that would put the second buyer in place to purchase the property if your deal fell apart. Typically, these offers are slightly higher. Your coucern should be to make sure all contingencies are met and deadlines are adhered to.
The "Eckler Team"
Anyway, READ the contract - have your agent and their broker do the same. Unless there is non-standard language in the contract, the problem will not be that he accepted 2 offers, it will be that he only has one house to sell. He can't possibly make good on both. If he breaches the contract, you could potentially sue for damages. Here again - us agents, we cop out and ask you to speak with an attorney.
As far as ethical - well, do you mean professional ethics for agents? Yes, the agent did her job. Do you mean SHOULD a nice seller do that, especially without getting you on the same page _ NO.
Broker / e-PRO