bryan,
While Ginseng plants with double tops are fairly rare, most seasoned Diggers have seen at least several or more during their digging career, while some others may have seen lots more! It would be hard to put a number on the occurrence but if I had to venture, I would say that it is close to 1 in 5,000 or possibly more. Some may feel or know that it is more or less (i.e. 1 in 3,000 or 1 in 10,000) and they are probably right, I just don't know. Now, my question is, are you sure that it is a double topped plant or did you unknowingly plant two seeds in the same hill and you actually have two plants growing out of the same hole? It would be easy enough to find out! You could gently dig down to the top of the root or roots and see if they are separate or joined.
Frank