Board Thread:Wiki Discussions/@comment-1512913-20140521221412/@comment-1512913-20140521222948

Hagoromo's Rinnegan split into Ashura and Indra's chakras ("body" and Sharingan) That alone is enough, since there being 2 Sharingans is impossible. It can't go Sharingan > Rinnegan > Sharingan, because the Rinnegan is the original stage of Sharingan. The antagonists haven't been attempting to create a new doujutsu, they recreate the original one. Rinnegan was already there before the Sharingan. The Sharingan is an evolution of the Rinnegan, by re-merging the sons' powers, the Sharingan essentially devolves back into its original form, the Rinnegan.