Ok make it another ''Confirm'' button for each of the traders after they already pushed it once, just to prevent things from getting scammed with this "faster faster, cancel, faster faster, in yo face".
Thing concerning duped items, it always can occur, be it on web or in a game. It is still safer for people to trade things, not just buy smth for rp, then trade extra stuff, just to get the thing you want from another person.