Seeing double


In order to do chapter 10’s challenge I copied the project folder and its contents to my desktop, per the suggestion in the book. I did the challenge using the copy, but when I returned to the original project folder, I discovered it was changed in exactly the same way I’d changed the copy (in hindsight this is funny of course). My question is why did this happen and how should I have copied the project to avoid this problem? Thanks in advance.