Error: L6200E: Symbol multiply defined

I have a code with following files:

- Extern_file.h

int variable = 5;

void function (void);

- Extern_file.c

#include Extern_file.h

void function (void){ variable++; }

- main.c

#include Extern_file.h

int main(void){ function(); return 0; }

When link, I got the error:.\Objects\Blinky_LEDS.axf: Error: L6200E: Symbol variable multiply defined (by Extern_file.o and main.o).
Im working over a code made from component designer (stm32F105) and migrating to nrf51822. This is only a simplified example.
Why can't I link it?
If I use CoIDE I don't have this problem.
I have readed another link about this problem, but I can't solve it.
http://www.keil.com/forum/21633/

Thanks.