Is it possible to duplicate eyes? This question contains spoilers for anime viewers and early manga viewers.


   When Nagato was revived with Edo Tensei, he had Madara's eyes. When Madara was revived, he had his original eyes too (Rinnegan, much like Nagato).


Is this exploitable? Can I repeat this cycle of transplant and revive to duplicate strong eyes? How does that work really?
Although never explicitly stated (till now), it seems Kabuto's upgraded Edo Tensei allows him to revive a shinobi with a mix of attributes from different portions of their lifetime. Shortly after showing the last coffin to Tobi, Kabuto exclaims that he had surpassed the Second Hokage and Orochimaru. It is very likely this is what he was referring to. 

It is possible to have multiple copies, and this has already been exploited with Itachi's Sharingan and Madara's Rinnegan. 

Both Itachi and Sasuke had Itachi's Sharingan at the same time. The revived Nagato and Tobi both have Madara's Rinnegan. If Itachi had not sealed Nagato, there would indeed be three copies of Madara's Rinnegan active at the same time. 

The Madara's Rinnegan case shows how it could be exploited multiple times. If Obito dies and is revived, while his Rinnegan is transplanted to someone else, you could then have four Rinnegan, which are all really Madara's Rinnegan.