About Error LNK2005

[lry]

I define some globle variable in my class,and In other class
I define this class's instance,When linking,
there show "error LNK2005,...already defined in *.obj",
why?
For example:
//myclass.h
DWORD dwMyData;
class CMyClass
{
public:
CMyClass(){}
};
//myotherclass.h
#include "myclass.h"


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