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It is impossible for someone to have two devil fruit abilities. Just as its impossible for 1 devil fruit to be shared with 2 people.

Once you have one, you're stuck with it. The main reason black beard has 2, is because of his type of fruit. It allows him to suck the ability from someone else, and take it.

That's the thing. No one said that it was because of his fruit though and people think that he might have simply stolen the fruit powers by bringing an ordinary piece of fruit and because once devil fruit users die, there powers go to the nearest fruit to the deceased user. Some people theorize that the Will of D and devil fruits may have a connection

Yeah might be something in those things but I haven't seen it yet. It's just generally Oda fooling around with the readers

Not really. Yes, some of his answers are joke like, but there's a lot of information in there that isn't in the manga or the anime.

Just like Lucky Roo's name. It was never mentioned in the anime, and though Benn Beckman was later on. We were given their names early via the sbs.

I think both those theories just seem off going with what Oda-sensei has said.

The power of darkness can nullify and absorb anything. Plus had it been a fruit, he couldn't have gotten the ability from eating it because you can only have 1 ability at a time. Eating another devil fruit would be pointless.

And I sincerely feel the will of D is more of the desire and dreams that man carry for adventure. But Will of D is still an unknown and only mentioned thing in the series.

um I'm pretty sure she means it is fact that they can not have 2 fruits
also in the CP9 one of the devil fruit users said if you eat a second fruit you die

I was wrong. They have just started hunting and it hasn't given any info on any powers they might have already gotten.

Okay the fact is that in the Enies Lobby Arc, Lucci and Blueno said that one's body would be generally destroyed by eating a second fruit. Blackbeard can use his Yami Yami no mi abilities to make darkness's gravity suck the devil fruits from corpses (from what I am guessing). So technically Teach did not eat the second fruit; also, he is just pretty much special and to top it all off, he is being pretty smart, so he uses this to his advantage. But as said, no, people will die.