Number of mallard eggs

A mallard has a nest in a box on the pontoon next to my canal boat in Roydon, Essex, and she seems to have been sitting on eggs since the 3rd April.

She got off briefly today and I counted 22 eggs!

There has been another female mallard that she 'allows' just in front of the nest box. I guess this to be a female from the sitting female's brood from last year.

For this many eggs I imagine both females have layed a clutch in the same nest.

Would this guess be right? I have never heard of this happening before.

Thanks.

Andy.