Duncan is a really popular EP. However, most of that traffic is just daytrippers but, Duncan can still fill up. There's no way to predict campsite availability. I think the site nearest the portage to Partridge Lake portage gets overlooked some. It's not a great site but, will work for smaller groups.
Also, if Duncan is filled up, Moss lake is a fairly easy portage away and you could see the campsite there from the end of the portage so, you wouldn't have to commit hauling everything across until verifying that site is available.
Another option, in which I'll all but guaranteed availabilty, is Partridge Lake. There are 3 lower quality sites located there, which is also just one portage away. However, the portage is fairly challenging and may be more than you're up for?
For more detailed information; check out our trip report from last summer.
Duncan Lake trip report