Ben and the Others were surprised by the crash. Ben had no idea who was on the plane, he sent two people, Goodwin and Nathan, to spy and pretend to be survivors, collect names, etc. I believe The plane was off course because it hit a time warp and was duplicated due to ...? Again, Ben had no idea who was on the plane.
SLIGHT POSSIBLE SPOILER BELOW?

??
In the final episode of season 3 in Jack's flashforward, Jack yells at the head of surgeons he should go get his father and if he is drunker than me... The point being, in the future Christian is alive.
How can that be? Christian died, had a coffin, etc. I believe that the Losties were duplicated in a time warp. The airplane Naomi talked about REALLY exists. The Losties died in an airplane crash, and their duplicates landed on the Island.
So the Losties are duplicates and Christian's duplicate is alive?
I think the key is Danielle. The French lady talked about a fever with her companions? I believe Danielle didn't catch this fever because her duplicate died in the accident.
What would happen if you got too close to your living duplicate?You people are all over this. Great thinking and conclusion-drawing. Does anyone think that a duplicate of the Oceanic 6 are going home but they are also staying on the island?