Double yolkers aren't necessarily a sign of really great health. Neither are they always a bad thing.
I can only tell you that my bluebelle lays enormous eggs, daily, since February, including quite a few double yolkers, and she is very sickly. She 'superovulates', occasionally up to four yolks being released in one day (which of course is not at all healthy for her), (resulting in, for example a double yolker in the morning, a soft shell at lunchtime and another soft shell in the evening), and has been to the vet several times because of complications which have almost killed her.
Our BO, on the other hand, who lays relatively small eggs by comparison, and only around five a week, is fabulously healthy and happy.
Others may tell you that the odd uncomplicated double yolker being laid is a sign that a hen is doing well. She certainly wouldn't be able to support double yolkers if she were malnourished, for example, so a seriously unhealthy or badly nourished hen wouldn't lay double yolkers.
You need to be sure that this girl gets what she needs to form these eggs properly - plenty of calcium for the shells and perhaps some extra protein in the form of sunflower seeds. Our girl Bella has to take prescribed calcium supplements, to ensure the soft shells are kept toa minimum.
