dude , this is very easily to explain and to understand , because if you think about it , it will make sense , i'll explain , you know that the black zetsu is madara's will , madara's will is exactly this: Body , Chakra , Power and HIS KNOWLEDGE. what does the white zetsu do ? he can transform himself into others person chakra when he touches to one person , right? and what does obito have
INSIDE him? he has a part of madara , to be exact half or a part of madara's chakra , and all that good stuff , what did you saw before obito became the juubi jinchuriki? you saw that madara was using obito and sacrificing obito to rinne-tensei madara , my theory and the most probable is this: since you read in the manga that for madara to be the juubi jinchuriki he has to be
ALIVE and since he was forcing obito to do it by using his own will(madara's will) , when madara was being resurrected and accomplish his resurrection he would be juubi jinchuriki , meaning that he was doing the seal for becoming the juubi jinchuriki plus doing the rinne-tensei seal (automatically) by madara's will and "rinnegan" because you saw madara with the rinnegan and doing the hand seal , also we can assume with this alone that we have 2 or 3 madara's "alive" meaning that we have 1 black zetsu , the other black zetsu that is inside obito and the edo-madara when in total is 3 , did you understood ?

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