Maybe Desmond did see Echo's death , but knew that there is no way to stop smokie, therefore could not help change that outcome . He has been on the Island a lot longer than the losties so he is bound to know more about all that stuff. If I were Hurley I'd ask him about stuff he knows...

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If I could wake up in a differant place , at a differant time , could I wake up as a differant person ? ? ? Tyler Durden