Look at it like this:
You are placed in a box. Moments later, you are told, "We have successfully made a copy of you. We are sending it home now. You must be disposed of."
Will you allow them to dispose of you?
This is the question being posed, not whether a copy will have no idea if it is the original. The point is that it isn't relevant if one is a copy. No one was moved, it's only that a second person now exists and killing either is murder of a unique person.
(Again, uniqueness is not a question of whether these people will think or react to situations in the same way, but rather that there are two different conciousnesses at play.)