FFI allows multiple calls to `pactffi_with_body`

[TimothyJones]

If you call pactffi_with_body twice with different arguments for the same interaction, the second one silently clobbers the first.

I think the ffi should either fail or at least log a warning in this case - otherwise we'll have to build that logic in every client of the rust core (which brings us back into the problem that having the core was intended to solve).