broody hen and soft egg, can anyone help please.

hi everyone, could anyone help please, i have two questions and need help!!!.
first, we are now getting 1 soft egg everyday, we have grit and oster shell at all times in their run, fresh water , they are feed on corn and layers pellets and let to roam free in garden half of each day. all seem happy, nothing wrong. cant find out as yet which one is laying the soft shell egg, but we have 5 nesting boxes, they all want to lay in same box, the soft shell egg gets broke and they are now eating it. help needed please.
also, i think we have a broody hen, showing all the signs. what do we need to do and how long will it last please. hoping you can help on both problems for me. enjoy the bank holiday folks xxxx