Broody hen?

I am fairly new to hen keeping - I have two. They are about 18 months old and been laying reasonably regularly for about 14 months. They don't seem to have moulted.
One now seems to be spending all her time in the nesting box in the coop - I even have to shove her out when I bring treats. Is this brooding? I have removed the china egg and the other hen's egg when she lays one. How long does it usually last? and how can I discourage brooding if that is the right thing to do?
She looks well - bright eyes, red comb and perky when she does emerge from the coop for the treats.
This one does tend to lay multiple soft shelled egs so I don't expect her to continue laying as long as the other hen - could this be connected?
Thanks for any suggestions
Joan