If you keep your expenses down, there's no reason that you can't save up for a down payment, and pay at least a little ahead on your student loans. 6.8% is high, so I'd definitely want that down sooner than later. Since you're paying 200 less in rent than you could be, I'd pretend you're paying that extra 200 a month, in addition to your savings. So put that 200 towards your student loans, and then keep saving for the duplex, or split it. The numbers on the duplex, assuming you have a good interest rate looks like it would work out. However if you're coming fresh out of college, you may not have a great credit score, and your interest rate may be higher than 5%. That alone will almost guarantee you'll be saving for a while.
This of course is all assuming you get a good paying job once out from school. Also, are you sure you want to buy right away? What makes you think you won't want to move in a few years? Or be offered a job across the country?
Also, make sure you keep an eye out for all paperwork from your student loans once graduating. I was sent paperwork that was explaining the interest rate and everything. And then I noticed that they sneakily said no response is required, but if you read on, you saw another line with something to the effect of: sign that you've read this and send it back to us, we'll lower your interest rate by half a percent (or something like that). And since my subsidized loans were all split between years, I got a few of those, but not for every year. But so all I had to do was sign my name, and I was able to take like 1.5% off the total rate. That alone will take your monthly payment from 425.80 (total paid of 51095.52) to 397.89 (total 47746.86). Hopefully they give you that offer.
Yes, that 425.80 is how much you'll be paying a month on your loans. If you haven't figured that into the fray, I'd make sure you do. As the duplex payment will be (depending on down payment) 700 - 750 a month. So with no renter in the other half (which is very possible), you'll be on the hook for about 1175 every month. Just somethings to keep in mind when doing your math.