We provide oyster shell in a separate small dish near their regular feed so that they can take it as they need it. It helps make up for the calcium they use to form eggshells. We have occasional soft-shelled eggs, even with the oyster shell available. Sometimes an egg is laid before it is fully hardened. This can be true especially if they are laying more than one egg per day. So far, nothing has been wrong and it’s been an unusual occurrence.
If you are not offering oyster shell, consider starting. If you are, but are mixing it into the feed, I would consider offering it separately, free choice, instead, so they can easily get the extra they need when they need it and not when they don’t. Too much calcium has a downside for ducks, though I’m not well-informed about the specifics, and free choice allows them to self-regulate their intake.
I hope these soft eggs will turn out to be an aberration in an otherwise healthy egg-laying life for your ducks. Our soft shelled eggs seem to come early in the egg-laying part of the year when their systems are reregulating after a winter break.