I totally understand where you're coming from, i have a friend that does exactly the same thing. She never works hard in anything and i try my best all the time, and she just borrows mine and copies it. She even takes my work without asking me and copies it, and then tells me just before the teacher asks if i did lend it to her it so she doesn't get into trouble. Of course i cover for her, but she doesn't deserve the grades she gets. I don't think you're stingy at all!