I understand how frustrating realtors can be. (we move fairly often) I'd calmly explain that you feel you are missing out on some housing options and that you do NOT want to see anything but duplex's. If after that he fails to step up, then like the Jo and Neil said... move on.

Sometimes it isn't the realtor's fault. I know we had a few people make offers on our house before we even put it on the market, so it's completely possible the same is happening with the duplexes you are seeing sold so fast. You may want to scout the neighborhoods your interested in and look to see if there are any for sale by owner as well. (just make sure you have the house inspected by a highly recommended professional)