If the eggs were taken I'd assume you wouldn't have found the broken shells in the nest? I've seen adult bids (oystercatchers) look for newly hatched chicks when they found their nest empty (eggs were taken by a skua). In this case there was a lot of looking around and calling the young birds by both parents, not just one.
Is it possible that the eggs hatched but something happened to the chicks so the mother returned for a second laying?
Thanks Marina! I guess it is possible that the ducklings hatched and then died somehow. But the eggs do look a bit attacked ( they are in bits). The mother duck left after some hours, but came back this morning to sit again, then peck around and apparently eat a bit of the eggshell. She then flew away to join the male who is on the rive.
In cases I have seen predators taking eggs they have taken them whole, carried them somewhere and ate them there. Unless the nest was vandalised by people (who scared mother bird off), of course. Did you just find broken shells or the egg contents as well? Have you got any photos of the nest with the broken shells?