Duck eggs possibly bad?

[Kkliegl]

We have a duck someone dropped off couple years ago and she went "missing" about a month ago but then randomly popped back up. I figured she was sitting on eggs so tonight since I was worried she was sitting on them in the field behind our house that's about to get chopped I watched her go back to her nest (thankfully not in the field).
After seeing her nest she's sitting on 5 eggs. Her eggs are usually white but they looked off so I endured being butter to sneak one to candle it. This egg along with the others all look like they have black spots inside. Candling looked like Close to hatch but couldn't see any movement. I only did the one but some of the other look worse with the black.
I have no idea how long she's been sitting on them but I'm guessing 3-4 weeks.
Does anyone know if this is normal or are these eggs bad?

 

[chelseachook]

Never experienced this, from what I've read - sometimes they can have bruises but be fine. Also if the duckling can't break the shell to pip, it can make a bruise. But the marks are all over the eggs so I don't think it is from that.
Do they smell bad?
I would candle them all and look for movement. The egg in candling picture seemed ok. But I wouldn't remove any eggs even if you do think they are dead because it is so close to hatch day.