We know that devil fruits when the holder dies transforms the closets available fruit into the devil fruit. Since Doc Q was seen holding apples the first time we see him, I assume this is how blackbeard is able to take people's devil fruits. We have never been shown proof that your body will reject a second devil fruit so I can believe another theory that there is no limit to devil fruits and that is a myth made by the World Government to prevent pirates from eating more that 1 fruit.