multiple construction blunder including c++ header record inline formula churned sources

i have c++ header record containing class.
i wish category several projects, bu i don't wish emanate removed library it, i'm putting both methods declarations definitions header file:

// example.h
#ifndef example_h_
#define example_h_
namespace test_ns{

class testclass{
public:
    vacant testmethod();
};

void testclass::testmethod(){
    // formula here...
}

} // finish namespace test_ns
#endif

if inside same digest i consolidate header some-more cpp file, i an blunder observant "multiple construction test_ns::testclass::testmethod()", while i put slight construction inside category physique does happen:

// example.h
#ifndef example_h_
#define example_h_
namespace test_ns{

class testclass{
public:
    vacant testmethod(){
        // formula here...
    }
};

} // finish namespace test_ns
#endif

since category tangible inside namespace, shouldn't twin forms equivalent? since slight deliberate tangible twice initial case?