I honestly can't tell you what to do, whether you should open the egg, or just remove it from the incubator and let the embryo fall asleep humanely. It's up what you feel like, what you can deal with. What I can tell you, if you consider letting it continue developing, is that the duckling will probably not live very long, if it makes it to hatch. I had a VERY delayed hatch with a chick once, if I recall she took about 5 or 6 weeks to finally hatch, which in itself was a miracle, but she was stunted and only lived 6 months.