Answer:
People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?
I saw the dead bring forth the living. I saw the living bring forth the dead. What did I see?
As I went over London Bridge I met my sister Jenny; I broke her neck and drank her blood and left her standing empty.
I am two-faced but bear only one. I have no legs but travel widely. Men spill much blood over me. Kings leave their imprint on me. I have greatest power when given away yet lust for me keeps me locked away.
Tear me off and scratch my head what once red is now black.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.