It's not uncommon, in a well-written code base, to see documentation on different functions or algorithms with where they came from.

This isn't just giving credit; it's valuable documentation.

If you're later looking at this function and find a bug or want to modify it, the original source might not have the bug, might have already fixed it, or might have additional functionality that is useful when you copy it to a third location that wasn't necessary in the first copy.