My ducklings were sneezing for a while, too. Turns out they were basically inhaling their duck crumble--now that I have switched to pellets, the sneezing has stopped. I gave them some apple cider vinegar in their water as an immune booster, too, just to cover my bases. Make sure that their water is deep enough that they can stick their heads in and submerge their entire beaks, also. Keep us updated!
I didn't go away completely. The heavy sneezing only lasted a while and now its much less common. I do think there's a respiratory issue. Just now, I heard a new sound. Kind of like a wheeze I guess. Very very similar to the squeaky sound a clown would make. Not sure from memory if its shoes or nose or maybe just circus but you get the idea.
They had a very long bath yesterday, but they're easily getting kind of like saw dust on them. Pretty sure its from the pine pellets. If I were to guess tho, it is actually the food, and water has been corrected. I think its only one, and they're probably safe, and its probably whatever got stuck before.
Figured it'd be safer to post an update than to potentially learn the hard way there is a danger.
P.S. I'm not sure why, but I've noticed in the past 2 days or so, more have been acting like they're choking a bit and trying to get food down. They're still on starter crumble since I haven't ran out. I figure in a few days they'll finish. They're 2 wk and 3 days old (avg, give or take a day for a few). I figure they should be okay still eating it an extra week.