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.
There's no real difference with what serde_codegen does, and what the compiler does for expanding macros. It's just expanding to source rather than raw AST.
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.