Multiple definition of namespace::variable even using ifndef

I know I must be doing something wrong here.

rank.h

#ifndef RANK_H
#define RANK_H
namespace mmi {
int chunk;
void rank(int my_rank);
}
#endif

rank.cpp

#include "rank.h"
namespace mmi {
//do something with chunk
}

main.cpp

#include "rank.h"
int main() {
    mmi::chunk = 1;
}

And the output of compilation;

g++ -g -Wall -std=gnu++11   -c -o main.o main.cpp
g++ -g -Wall -std=gnu++11   -c -o rank.o rank.cpp
mpic++ main.o rank.o  -o main
rank.o:(.bss+0x0): multiple definition of `mmi::chunk'
main.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
Makefile:12: recipe for target 'main' failed
make: *** [main] Error 1

My understanding is that header file is being included multiple times. But I was expecting to correct this issue by using the #ifndef.

So, may I ask what's going on here?