Why it's valid to include a header file twice in c++?

Question

#include \"DLLDefines.h\"
#include \"DLLDefines.h\"

The above actually passed compilation, but why?

Answer 1

It's called an include guard.

#ifndef GRANDFATHER_H
#define GRANDFATHER_H

struct foo {
    int member;
};

#endif

Quote from Wikipedia:

In the C and C++ programming languages, an #include guard, sometimes called a macro guard, is a particular construct used to avoid the problem of double inclusion when dealing with the #include directive. The addition of #include guards to a header file is one way to make that file idempotent.

See link above for more information.