For what it's worth, you only need one file to be included. Any other files will be included and translated as necessary, so the only file that needs the .in extension is the root of the serde types.
The idea would be that these types would be put into a library (or just another module) that's used by the rest of the app. There wouldn't be duplication of these structures. It's not ideal, which is why we are working on stabilizing the compiler features to do away from codegen.