We just got back from a trip where we stayed on Duncan.
Daddy daughter Duncan dayrrips While Duncan was full most days, The sites on Rose were generally open on the days when we passed through. Also, Moss lake is an easy portage away and I don't believe anyone was camped there the whole time we were in the area. And, if those options don't pan out, I can just about guarantee there will be campsite availability on Partridge lake.
Of course there's no 100% guarantee but, I'm confident you'll find something. While Duncan is usually buzzing with canoes coming and going, most don't camp there. Have fun!