http://i3xi5qxvnvtjk3am4vkw3xvfecjitic3brox4tfgl6mxqyzctqudscqd.onion/tor-spec/creating-circuits.html
When an onion router receives a CREATE/CREATE2 cell, if it already has a
circuit on the given connection with the given circID, it drops the
cell. Otherwise, after receiving the CREATE/CREATE2 cell, it completes
the specified handshake, and replies with a CREATED/CREATED2 cell.