Possible to duplicate current java stack using JNI

Answer 1

No. The stack effectively goes away when the method is compiled. The JVM has no way of compiling native code. So even if you did try to directly manipulate the stack, it would change format (and use registers) on the fly.

You can reasonably easily duplicate the top four slot of the stack (using dup2_x2), but any further and you'll probably need to use local variables.