I have 15 6 month olds, starting to lay, nice eggs getting larger. Today I found a large double yolk with NO shell. Huh?!
its somewhat common for this to happen when they first start laying, or after a period of molt. keep an eye on her and make sure she doesn't get egg bound. offer some oystershell or other calcium source free choice. if she stops laying and wings start drooping, then its time to worry.