Okay just a quick thought... i was thinking how BB can have 2 DF powers.... and i came up with a (possibly stupid) theory. Do have a read and give me your thoughts in the comments below.
The Yami Yami no Mi:
The yami yami no mi is a devil fruit that can take something into a void and later spew it back out at the user's conviniece. So maybe BB did not actually absorb the Gura Gura no MI but he actually absorbed the ability from WB's corpse. Bear whith me while i try to explain my reasoning; when BB touches a DF user, their powers are nullified, what if he is actually absorbing the power? but then since the user is alive, the power goes back to it's original user's body. Now I asssume that there will be a small transitional window where the user dies and the DF power leaves his body to go back into circulation. So after BB killed WB, he used his DF power to absorb the Gura Gura no Mi's ability. And he waited for the "transitional period" to be over so the DF power would not return to the user's body. So the DF power stayed within the yami yami no mi's void. I think that when BB dies and the yami yami no mi goes into circulation again the gura gura will also return to circulation.
SO what do you think? do leave your thoughts in the comments below :)