This will be great if you can update the existing Song in Mixdown, So when add a song into the ShowPage there is generated I should have an option to update and not create a new song/stems. Im echoing what the previous person said, you must currently, delete the wav file from the show page and re-insert it manually, otherwise if you try to export the song into the show again, it creates a new song in the setlist rather than updating the existing one and if im right some how save it and take up space. I think In the project page you got it right to go back Jinsong page , update it and it automatically update it in the project page when you save the updated one.
I think you get what im saying.