I'm not an expert, but I wanted to keep your thread alive, so I thought I'd chime in. But remember, opinions are like...well, anyway. lol
I'm a big proponent of having one place free and clear! That way, if all else goes badly in the world, at least you have a place to live! It's better to be broke than broke AND homeless. I think you should never put another person's dime on your duplex...unless you end up buying something else and have it free and clear later, then you can at that time. ;-)
That being said, I vote for a SFH. It's just my opinion and many people do well with town homes, but for me, I don't like the idea that it increases some of my risks. If their place catches on fire, mine probably will, too. If they are gross and have tons of roaches, my place probably will, too.
I think you should keep the duplex as is and buy a SFH. :-D Then again, if you know anyone good at the stock market, consider putting some in that for a bit of diversification. I have about 1/3 of my money in property and the rest in stocks and bonds, though I'm looking to make it more half and half. :-)
Good luck with whatever you decide! But, after this thread is dead, I'd love to know what you plan to do. :-)