Try to imagine the following: you wake up in a hospital bed, with no recollection of how you got there. You look over at the bed next to you and see a person who looks identical to you, down to your birthmarks and scars, staring back at you. You talk for a moment and realize that you both identify using the same name and seem to share childhood memories. The other person also has no recollection of how they got there.
Please someone explain to me, how you would argue either that you know you are the original or argue for why you can't possibly have that knowledge. While forming your argument, please also explain where "you" are and who "you" are.
You may want to consider issues such as what it means to Know what your identity is, what identity means, and whether it even makes sense to claim that your identity even survives.