Shanks stole it because its worth a lot of money. read the buggy and shanks flashback and you will se that buggy that doesnt want to eat a DF still wants it because of the value of it.
no, Angel Emfrbl did not use a quadruple negative. (and in case you think i just used a double negative "no Angel Emfrbl did use a quadruple negative" doesnt make much sense now does it?)
none of the negatives affects the same part of the sentence. they dont counteract each other as is the case with double negatives. (although i think the part after the comma is missing a few words)
if he had said "no two people with the same Devil Fruit can't exist" that would have been a double negative as the "can't" affects the "no" in "no two people"
i highly doubt that vegapunk gets his hands on all the fruits shown in the DF compendium sanji is talking about. if that was the case all DFs would imo have gone to the marines instead of ending up in the hands of pirates such as blackbeard, absolom and possibly brook. (the fruits that is most likely to havebeen identified before they were eaten)
Also there is no proof that GDR had any DF. for all we know his cuffs were wooden when he went to be executed. if he had any DF he would have needed seastone to neutralize any such power imo.