One or more multiply defined symbols found

Put the definition (body) in a cpp file and leave only the declaration in a h file. Include guards operate only within one translation unit (aka source file), not across all your program.

The One Definition Rule of the C++ standard states that there shall appear exactly one definition of each non-inline function that is used in the program. So, another alternative would be to make your function inline.

Make the function inline or declare the function in a header file and define it in a cpp file.

inline int DebugMessage(const char* message)
{
    const int MAX_CHARS = 1023;
    static char s_buffer[MAX_CHARS+1];

    return 0;
}

EDIT:

As a comment by Tomalak Geret'kal suggests, it's better to use my latter suggestions than my former and move the function's declaration to a cpp file.