Mine do that to! I have a broody who sits in the box all day. There are 2 other nest boxes right next to that one. But I'll find 2 other girls in the same box with the broody and the others empty.
I know! I have 6 empty nest boxes and 5 chickens piled into 2 nest boxes together! No matter how many times I try to rearrange them they all pile back up again to lay.
I give up!
Mine do that too! I've got 7 hens and 4 boxes and they all want to be in the same one. I witnessed 1 hen jump onto the back of 1 that was already in the box and lay an egg on her! It was very interesting & wierd! All of the other boxes were empty. Who can figure out the chicken brain?
This is how I got into SOOOO much trouble this last fall. I had a broody hen in the box and the other girls would lay and when I went to lift the broodies butt she had 14 eggs under her and she started with 4 !!
I had to take them all out and put them into the incubator and close the nest boxes for a few days.
They started laying other places, but the brood hen was broke of it by then.
I only hatched out 2 chicks from that, the rest were taken by my DW and that was that. We candled them an there was no movement or they eggs were cleared.
Adding another doesn't mean they'll use it. I had 4 nest boxes in my small coop for the Delaware flock and 4 in the big coop for the RSLs... out 12 hens only 1 lays her egg in the Delaware coop. Lately I've been finding 4+ eggs in one box, a few in another and 1 or 2 with none.