Dogen's Fake Knife by Engberg

So this is a theory concerning the dagger that killed Jacob, and the dagger that didn't kill MIB.

As we see in The Incident Part 2, there is a dagger that can kill Jacob. Well, we don't really know that it's the dagger, but for the sake of this theory, I'm going to assume it is. Now, we later see a special dagger with Dogen (the same looks as MIB's dagger?), and supposedly it can kill MIB. However, I believe that Dogen thinks this is the dagger that MIB had, that killed Jacob. I believe this dagger can kill both Jacob and MIB.

So I think that somewhere in time, MIB created a fake dagger to trick Dogen and/or Jacob to think they had the real dagger. So all this time, Jacob thought MIB had no way of killing him, but this whole time, MIB has actually had in his possession, the very dagger that killed him. I think he had to have someone special use it, though, and it turned out to be Ben.

So this would explain why the dagger didn't damage MIB at all when Sayid stabbed him. For this theory to be correct, it would mean that MIB got his hands on the dagger very long ago, because we can see that he has it in his possession in Ab Aeterno. I don't recall us seeing Richard returning it to MIB, though, so maybe Jacob took it. This would mean that MIB got it back again later.