If need be you will just have to remove the invader and house her elsewhere wether she likes it or not....some flocks can exist quite well with multiple broodies, in other flocks it is a total drama club....and drama clubs result in poor hatch rates and unnecessary chick deaths either through broken or neglected eggs or (less frequently) chick attacks.
The thing is that it is up to us as the flock keepers to recognize what the situation is and to remedy it...in a wild or open environment the hens would have room to spread out and seek isolation, in coops they do not, so we have to provide artificial barriers to give them their preferred (or needed) level of privacy.
Either move some of the hens or build a fenced area around their individual boxes to allow them to sit in peace, simple chicken wire can create an effective barrier.
If you move a hen and it breaks her broodiness then just wait until she goes broody again and move her earlier in the process...