Mallard Duck Made a Nest in My Front Flower Bed?

Last year we had a mallard duck that laid eggs and sat on them for a couple of weeks. After she did not sit on them for a few days, we discovered 2 eggs left in the nest, but the rest were gone. There were no egg shells like something had taken them out of the nest. The two remaining eggs did not hatch.

We do have opossums, raccoons, and fox around so it may have been one of them. She is back again this week (Indiana) which I find strange because something took her eggs last year. 1st question - Why would she make the nest in the same spot if something ate her eggs last year. 2nd question - What can I do protect them this year?