This is my theory, It can't be a madara clone because when madara was brought back and was talking to kabuo madara ask if HE (other "madara) knows about the plan. Meaning if it was a madara clone then the question would not exist. Madara knows about Orochimaru so that gives us a timeline on how long he lived.
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. My guess is that Tobi was desciple of madara. My theory is is that obito was rescued by madara and madara had his lil collection of Hashirama's dna where he used it to recreate obito and was named tobi which was the one of the form of zestu. Madara was the first. Madara was the first form of zetsu after the fight with Hashirama he remade himself with Hashi's dna. Later when the years when on, Madara had taught tobi the uchiha way. Somewhere Orochimaru came into the picture with his obsession of collecting dna to create the perfect body for himself. He and madara create akatsuki with tobi being introduced as madara. Madara was the one who attacked the village and was defeated by the 4th. After the 4th sealed his powers and was no longer able to use the sharingan to control the kyuubi, it was up to tobi to carry out his will. Madara being Obito (tobi) closest friend, he killed madara giving him the MS and taking the rennegan. THIS IS A THEORY, so dont be all defensive and whatnot