Found a really giant egg in the nest box this afternoon. The egg in the middle is second year Khaki Campbell and we don't have geese. The egg has a really thick shell.
I've had chickens - usually pullets - in the past that have laid ridiculously large eggs. Triple yolkers, actually. They didn't even fit in the carton well. Maybe your duck laid an extra large jumbo egg? Do they do that? I know nothing about ducks, I'm just guessing...
Thanks for the input Bobbi-j. I found this video about a pullet in China that laid an egg within an egg. I'm going to crack ours after our scale comes from Amazon so we can get a weight.