> There wasn't any good path toward refactoring this code to reduce the duplication. Most of the C++ code referenced the protobufs directly, and even if most of the data in one had the same names as in the other, you couldn't just interchange or combine them.

That makes it sound like the problem is more of a spaghetti mess than duplication.

But I think the advice to copy something when you need two versions is supposed to be applied to specific functions or blocks or types. Not entire files. Then it wouldn't have duplicated the geographical code.

It's also important to have a good answer to how you'll identify duplicated bugs. I'm not sure how to best handle that.