Symbol "is multiple defined"

Well, I'm baffled here. My code compiles just fine, but it fails at linking.

I've set up my code so that I have multiple C files, each with their own headers, and the main C file including all the headers. Each header is wrapped in #ifndef, #define, #endif statements to prevent duplication. It looks like my code should work, but it doesn't. I keep get Symbol "is multiple defined." Anyone have any recommendations?

Never mind. I'm a noob at C projects and hadn't learned the art of the "extern" command.