I'm quite not sure how one would go about empirically measuring something like that -
at least in the general case. It might be an interesting research topic. So no,
unfortunately I don't really have hard evidence for this.
I just know that of all the C libraries I've looked at, in my personal experience it
seems to be a very common phenomenon to copy or reimplement code that in Rust you would
just import and re-use.
It's just a pattern that one notices frequently when it comes to C libraries,
especially crossplatform ones that can't rely exclusively on the existence of a
Linux-like package manager.
If you want specific examples, the ones that pop to mind are:
* zchunk and deltarpm both reimplement / "bundle" multiple different hashing
algorithms
* libcomps implements about 4 different relatively common data structures
* GTK appears to contain a bundled, forked copy of the CRoaring library