**Spoiler alert**
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I figured Peter was a clone/experiment until the finale cleared it up. My wife thinks that Walter killed 'himself' in the other world to get another Peter. Thus, he went insane.
Did anyone else pick up on the blatant "Dark Tower" reference when they were figuring out where the baddie would strike next? Dude, they even used the word "thin". Lost is already suspected of being involved with the Dark Tower series, and now it appears Fringe is too.
Just so everyone knows, Stephen King sold the Dark Tower movie options to J.J. Abrahms and his Lostie associates for the ironic sum of... $19.