It is unlikely that you have found a duck egg, even if you have had one regularly visiting your garden. This is because female ducks, like most birds, sit on their eggs to keep them warm until they hatch. Some people believe that ducks will bury their eggs to hide them from predators. However ducks usually tend to stay near large bodies of water and lay their eggs in nests near the water but they will most likely stay with or near the eggs to keep them warm and guard them until they hatch.
Two animals that do bury their eggs are alligators and turtles, but it seems highly unlikely that either of these two animals will have buried their eggs in your back yard.
The most likely possibility is that a fox has moved the eggs into your garden and buried them. Foxes are renowned for stealing eggs from birds and ducks and carrying them away from their mother's and burying them elsewhere so that they can come back for them later. Unfortunately, eggs are also one of a fox's favourite things to eat. The foxes will gently pick up the eggs with their mouth, carry it to a safe place, dig a hole, place the egg gently carefully inside and then come back later to eat it.
There is not much that you will be able to do about the egg so if you find an egg or see a fox burying eggs you should just leave it alone. Even if you were to get the fox away from the egg, you have no way of knowing where it came from and cannot possibly return the egg back to its mother.