We have 1 month old Fawn and White Indian Runners. Will they mate together? And would a diaper be sufficient to prevent it or would they need to be separated?
They will probably mate when mature. If you do not want more ducklings you could just collect the eggs she lays. Well, she will lay the eggs whether they are separated or not just like a hen would. Indian Runners are not a very broody breed, so it is unlikely that she would incubate the eggs anyway. If the issue is you just find it embarrassing for the ducks to engage in brief mating behavior, well, I don't think the diaper will prevent them from at least trying to mate.
Healthy? Depends upon any deleterious genes in the parents. Because they are closely related, the probability of deleterious genes being paired is increased. I doubt you would have any problems for a couple of generations, but you never know when mutations will occur that could be paired. It is a random chance problem.