If it was me. I would help it out. Just be very careful and go very slow. I candled mine so if there was a vein leave it and go around it. If the cord is still attached leave it. It will fall off in a day or so. Good luck
Ducks sometimes take very long to progress from pip to completing the hatch, so if the duckling is o.k. in there don't rush to help it yet. Just make sure the humidity is high enough in the incubator to prevent the inner membrane drying out and trapping it in the shell. If it doesn't progress over the next few hours and shows signs of being in distress you can help it zip by very carefully chipping away bit of the shell with a clean pair of tweezers, working your way from the pip hole around the egg. Don't tear the inner membrane which covers the duckling, just remove bits of the shell. Take a wet Q-tip, dampen the inner membrane and check for active blood vessels. If there are any visible active vessels stop what you're doing and replace it in the incubator. Continue after 2-3 hours and repeat: Check for vessels and if there aren't any, check the area at the duckling's belly, where the yolk sac is. Make sure there are no active vessels and no sign of yolk there before removing the rest of the shell. If you do see signs of yolk or vessels down there, wrap the egg in a clean damp cloth and leave it again for another 2-3 hours. Good luck!