My ducks lay a good egg and a soft egg daily, what is causing this?

[BeasMom]

Spinach and bread are definite no-no's.

I have a house duck that lays regularly and we give her free choice oyster shells and turkey grower pellets. She's about a year old and for the first time this evening, she laid a soft egg with no solid shell. It's her second egg today, which is not normal for her. So if anyone has any thoughts or ideas, please share!