It's not common with most of the breeds. I believe that there are some strains of the heritage breeds (particularly the smaller breeds) that have been breed to produce longer laying birds so that there are eggs to sell/hatch all year.
As to your second question..will it hurt them. No. However they will have shorter years of laying. Poultry are born with XX number of eggs inside. Like a human woman. Once those eggs have been used up...that's all there is. You do need to keep the protein levels up for a hen that is laying into fall/winter so she doesn't lose weight, as well as calcium. They put a LOT of their own protein into building the eggs (and calcium, obviously).
You will probably see them go through a molt soon, tho.