Broody hens do not lay eggs. Other hens will lay in the nest with a broody and sometimes a broody will carry an egg back to her nest. If she laid an egg, she is not broody. If another egg showed up it does not mean she laid it.
Once a broody starts to set on the eggs, the eggs start to develop. If an egg is added after the others start to develop, that egg will not hatch before the hen leaves the nest with the chicks that have hatched.
Another problem. A broody hen does not eat and drink a lot. She will lose quite a bit of weight while she is broody because she is not eating much. If she were also laying eggs that are about 3% of her body weight when she is weakened from not eating much, she would be putting her life at risk from malnutrition. She could not survive being broody if she were laying.
Mother nature programmed broody hens to stop laying for some very good reasons.
My test to see if a hen is broody is that I have to see her spend two nights straight on the nest instead of in her favorite roosting spot. I've had several that spend a lot of time on the nest, make that broody clucking sound, cop a real bad broody attitude, but not spend two straight nights on the nest. The hormones may be acting uop, but she is not broody enough to hatch eggs.