Spoiler alert from Marine Ford Arc !!!
since until now (as far as I know) there has not been an explanation of how exactly Blackbeard managed to have the Yami Yami No Mi and the Gura Gura No Mi at the same time, how do you think he managed to have two Devil Fruits at the same time ? (If there is already an official explanation and I don't know it, sorry for not knowing it)