Hi,
Mating doesn’t always mean more eggs. But you are right, it’s not good for her to lay more now. Do I understand they are feeding one chick so far? And more eggs may still hatch if it’s just been 18 days. It’s a difficult situation. If you can tell any new eggs from the old, then I would remove them to keep them from ever developing. Then as soon as the chicks leave the box, take the box down so the parents will focus on weaning the chicks. Do not put the box back up for at least 6 months so the parents will rest from breeding. Often a pair does this if they are too young. Are both of your birds at least 2 years old? Younger birds are usually more interested in mating than caring for eggs and chicks. If your birds are young, then once you remove the nest box, I would not let them breed again until both birds are well over 2 years old.
Thank you for asking Lafeber,
Brenda