I didn't really ask for a lecture on doing home electrical work. I am really more interested in structural issues. I am quite aware of sizing cable, etc. If I had existing professional penetrations, I wouldn't always gauge anything by that. I've seen lots of "profs" bad work, too. I'm pretty sure my general knowledge, help from a few pro's on the web and my electrical engineering degree hanging on the wall, will enable me to turn off a breaker and extend a basement lighting circuit so the corner isn't dark.
All that other info would have been nice if I had asked a question about it. However, you should qualify the 'duplex' breaker statement by checking to make sure your load center allows duplex breakers. Some don't. Maybe you should call a pro and have them check yours out. We wouldn't want your house to burn down.