Preprocessor directives

Hello together,

I'm coding a simple UART-Application on the MCBSTM32 eval board and have problems with the preprocessor directives. I studied the whole documentation here and I'm pretty sure I'm coding in a common way, but still I can't compile my program. These are the very first lines of my main header file:

#ifndef ___MAIN_H_
#define ___MAIN_H_
#endif

#undef EX

#ifndef ___MAIN_C_
  #define EX extern
#else
  #define EX
#endif

And my variables a coded that way:

EX unsigned int ui_TxFrameCounter;

When compiling the program, I get the error code "L6200E: symbol multiply defined" for my variables, because of including the header to other files. I want to declare the variables "extern" for the other header files. Could somebody help me? Would be nice.
Geetings