In ducks, inbreeding for a few generations is harmless. Some commercial breeders sometimes breed closely related ducks to promote a specific feature (this is known as linebreeding).
You should be careful when mating siblings. If siblings continue to mate for more than 3-5 generations, deformities can become prevalent and there can be growth slowness in offspring.
Inbreeding is the mating of closely related ducks. In contrast, crossbreeding is the mating of ducks that are completely unrelated and often belong to different breeds.