dual agent can be a good thing.

As seller, you don't have to forbidden the dual agent. Most of agent will put the seller's interest first before the buyer's. The dual agent have more power to make the deal happen (the seller has the final say to make the deal or not). As a buyer, a dual agent won't sacrafice seller's interest, it just make the buyer beat the other bidders (which is good for the seller) to get the deal.