$ cat a.h 
extern int f()
{             
        return 5;
}                
$ cat 1.c 
#include "a.h"

int main()
{         
        void g();
        g();     
        return 0;
}                
$ cat 2.c
#include "a.h"
#include <stdio.h>

void g()
{
        printf("%d\n", f());
}
$ gcc 1.c 2.c
/tmp/ccrAqQEC.o: In function `f':
2.c:(.text+0x0): multiple definition of `f'
/tmp/ccQGjSBs.o:1.c:(.text+0x0): first defined here
collect2: ld returned 1 exit status