@DollarScholar:
To be fair, we dont know for sure why Blackbeard can have 2 DFs. There is no reason to believe that Marco is somehow an "expert" on the subject. It could very well have something to do with the will of the D. Blackbeard having a "Abnormal Body" would be a piss poor exuse to throw everything we know about DFs out the window just to create a strong villain, and Oda is sure as hell better than that. Im pretty damn sure we are going to get a more in-depth explanation.
A popular theory among fans seem to be that "The will of the D" allows you to consume two Devil Fruits and live, but only for a prolonged time, and this is how Gol D. Roger managed to conquer the New World (2 devil fruit powers) and also what caused his "fatal illness".
It may be a popular theory but it's super wrong. The 5 Elder Stars have already said there is nothing noted in history regarding a person eating 2 Devil Fruits.
Speaking about Black Beard in Vol 60, Birth Mark Guy says: "There is no historical precedent for a man who's eaten two Devil Fruits."
You'd think someone would remember that the most famous guy to ever live basically could use 2 fruit powers. There have been so many D's named and unnamed(Like Saul's entire family) I'd like to think they would have stumbled upon it by now. Teach had some understanding about himself that gave him the confidence that his body would not be destroyed by the Quake Fruit. That confidence in no way came from the Will Of D. because when White Beard mentioned it to him he seemed to be clueless on the subject.
Marco is no expert but they were on the same crew together for decades, they knew something was different about him. What ever "disorder" or "condition" Teach may have, I think it's been set up just fine to break an iron clad rule for one major exception.
Luffy is NOT eating the Flame Flame Fruit.