Duplicate Symbol XCode g++-4.2 - c++

Question

OKay, so I'm trying to set some global variables that can be accessed by the rest of my program by including a header file. However, XCode is telling me that I have duplicate symbols. Can anyone help?
Error: Duplicate symbol _ArrowKey in /Path/to/MKDBControlInterface.o /Path/to/main.o
main.h: // The global variables to be accessed...
#ifndef _main_h
#define _main_h
#include <map>
std::map<int,bool> ArrowKey;
#endif
MKDBControlInterface.h:
#ifndef _MKDBControlInterface_h
#define _MKDBControlInterface_h
#include <map>
#include <GLUT/glut.h>
#include "main.h"
#include "MKDBApplication.h"
class MKDBControlInterface {
public:
MKDBControlInterface( MKDBApplication& App )
: m_App( App )
{
glutSpecialFunc( SpecialListener );
glutSpecialUpFunc( SpecialListenerX );
ArrowKey[GLUT_KEY_LEFT] = false;
ArrowKey[GLUT_KEY_RIGHT] = false;
ArrowKey[GLUT_KEY_UP] = false;
ArrowKey[GLUT_KEY_DOWN] = false;
}
~MKDBControlInterface(){}
void static SpecialListener( int key, int x, int y ){
ArrowKey[key] = true;
}
void static SpecialListenerX( int key, int x, int y ){
ArrowKey[key] = false;
}
private:
MKDBApplication& m_App;
};
#endif
main.cpp
#include "main.h"
#include "MKDBApplication.h"
#include "MKDBControlInterface.h"
#include "MKDBRender.h"
int main( int argc, char *argv[] ){
MKDBApplication App;
MKDBControlInterface Interface( App );
MKDBRender Render( App );
return 0;
}

Answer 1

In main.h you need to declare ArrowKey as
extern "C" std::map<int,bool> ArrowKey;
and in main.cpp after the includes you should define it:
std::map<int,bool> ArrowKey;
BTW, I would also replace #ifndef/#define/#endif with #pragma once in the headers.