The Doctor and Locke!!!!
I'm going to try and draw a comparison between the doctor on the boats death and Locke's unexpected, unknown, and apparent suicide. The doctor died on the island long before he died on the boat which leads me to believe that a similar ending has happened with John. Island time, John is leading the others. Off island time, John is dead. One of these times will force the other to happen much like the doctors death. The doctor washing ashore lead to his death on the crater. Seeing John dead on main land will eventually lead to his real death on the island. I know there are a lot of thoughts out there to having 2 lockes but that is not possible since there has been no experiment or visual evidence suggesting 2 copies of anything ever existed. So like I said the time lag or the time forward will lead to his inevitable demise.