title: OpenGL Image (GLI) : one or more multiply defined symbols found
body: <p>I'm trying to set up a class that uses GLI but I can't get the library to work even with simple code.</p>
  
  <pre><code>-----------------------------------------------------------------------
                                   header.hpp
  -----------------------------------------------------------------------
  #pragma once
  
  #include "gli\gli.hpp"
  
  -----------------------------------------------------------------------
                                   source.cpp
  -----------------------------------------------------------------------
  #include "header.hpp"
  
  -----------------------------------------------------------------------
                                   main.cpp
  -----------------------------------------------------------------------
  #include "header.hpp"
  
  int main(int argc, char* argv[])
  {
      return 0;
  }
  </code></pre>
  
  <p>I never had a problem like this before but it seems like I can't include a header that includes the library itself in my main.cpp when it's already included in another .cpp file. I can include the library directly in my source.cpp instead of my header.hpp but I don't think it would be a great solution.</p>
  
  <p>The Visual-Studio build output says : </p>
  
  <blockquote>
    <p><strong>fatal error lnk1169 one or more multiply defined symbols found</strong></p>
  </blockquote>