So, I've got to say that Kishi has outdone himself this time. Hashirama and Madara were reincarnations of Ashura and Indra. Problem is, so are Naruto and Sasuke. If they all have the same soul, how are Naruto/Hashirama and Sasuke/Madara alive at the same time? Of course, the obvious out is that they only host Ashura and Indra's chakra instead of their actual souls.
But that creates a new problem. It's more or less implied that the reason Madara and Hashirama (As well as Naruto and Sasuke) are so OP is because they have that chakra. Yet Madara died, and Indra's chakra passed down to Sasuke. Which means when Madara reincarnated, he shouldn't have possessed Indra's power any longer. Yet he's more overpowered than ever, even before he became the Jinchuuriki. Same with Hashirama and Naruto.
But that creates a new problem. It's more or less implied that the reason Madara and Hashirama (As well as Naruto and Sasuke) are so OP is because they have that chakra. Yet Madara died, and Indra's chakra passed down to Sasuke. Which means when Madara reincarnated, he shouldn't have possessed Indra's power any longer. Yet he's more overpowered than ever, even before he became the Jinchuuriki. Same with Hashirama and Naruto.