multiple definition of a function

[nikosft]

I want to extend an open source project. Inside the project there is a header file named inet.h which has the following structure:

#ifndef __IPV4UNDERLAYCONFIGURATOR_H__
#define __IPV4UNDERLAYCONFIGURATOR_H__
class foo{
method1();
}

bool method3(){
//method implementation
};
#endif

I have created a header file name extinet.h within which I include intet.h and it looks like this
[...]
include "inet.h"
class foo2ublic foo{};
[...]

When I try to compile the project I get a multiple definition error for method3. Is there a way to avoid this error without modifying inet.h (e.g, without declaring method3 to be inline)?

Thanks