Given the number of people who commented here with their own stories I wonder if it actually is rare.
I imagine you only find out if you're undergoing x-rays, operations and similar.
If there are no obvious side-effects there could be a lot more flipped-people out there.
Doesn't almost everybody receive x-ray, ultrasonography, abdominal palpation, stethoscopy, ECG or surgery today? Does it happen often that a person grows up never ever examined by a doctor closely?
Probably as developmental defects go it has to happen really early and be just right or everything goes sideways.
Why is this even a defect? Why do the body parts have to be located the way they usually are and not mirrored? Why dos the body development orientation take the right-handed or the left-handed direction randomly?