Duck egg craziness!

[duckmama618]

We recently lost a Welsh duck to what we think was a respiratory problem. Today, we cracked open this egg, but not sure from what duck or what day exactly. We have about 3 ducks laying. Does anyone think it could have been from the sick duck or is this something that happens? We want to avoid giving out eggs that look like this!
Thanks for any input

 

[Hillaire]

is it possible that this was an egg you missed and they started to incubate?