From time to time your duck may go broody. In some ducks this maternal instinct is stronger than others and it can happen at any time. It is quite easy to spot because the broody duck will simply sit in the nesting box (or flower pot!) and refuse to budge. She may also make a peculiar growling noise if disturbed and become quite aggressive. However, unless your duck has been near a drake within the last 7 days the eggs will not be fertilised and will never hatch into ducklings. If you are not removing the eggs everyday there is more chance that a duck will go broody.
What you should do
If you do nothing, your duck will stay like this for up to 3 weeks (the incubation time for eggs). It is not necessarily a problem but it will prevent your other ducks from laying in the nesting box. You can remove her from the nest and block the nesting box so that she cannot get in. Be brave, open up the egg port and lift her off. After a couple of days she will lose the urge to sit on the eggs and you can open up the box again.