LNK2005, "already defined error" linker error in MSVC2010

I am trying to implement a test project using the Point Cloud Library and OpenCV with multiple files. When I try to compile, I get the "already defined error" message. Probably I'm doing something stupid that cannot realize for some reason - I tried out a couple of solutions found here, none of them seemed to be help in my case.

What I have:

A libs.h file, where I load the lib files (in Project properties, I only set up the .lib paths and load the libs "by hand", like the headers):

#pragma once

#ifndef PCLTEST_LIBS
#define PCLTEST_LIBS

#ifdef _DEBUG
  #pragma comment(lib, "pcl_apps-gd.lib")
  #pragma comment(lib, "pcl_common-gd.lib")
  // a bunch of other debug libs
#else
  // the release libs
#endif
#endif

A main file from which I basically deleted everything at this point to debug:

// load the libs
#ifndef PCLTEST_LIBS
#include "libs.h"
#endif

// pcltest includes
// if only this first one is #included, everything is OK
#include "opencvOperations.h"
// #including this one causes the error
#include "files.h"
// these ones are not working also
//#include "cloudOperations.h"
//#include "visualize.h"

// c++ headers
#include <stdio.h>
#include <string>
//#include <sstream>
//#include <iostream>

void writeInfo()
{
    // some std::cout calls
}

int main( int argc, char* argv[] )
{
    writeInfo();
    // this function is in opencvOperations.h and works OK
    pcltest::openLena();
}

Then I get several error messages in my main.obj that some (PCL related) symbols are already defined in files.obj. I use PCL related calls both in opencvOperations and files, the first one is OK, the second one does not work.

Edit: To add more detail, my files.h header:

#pragma once

#ifndef PCLTEST_FILES
#define PCLTEST_FILES

// pcl headers
#ifndef PCL_COMMON_H_
#include <pcl/common/common_headers.h>
#endif
#ifndef PCL_IO_FILE_IO_H_
#include <pcl/io/file_io.h>
#endif
#ifndef PCL_IO_PCD_IO_H_ 
#include <pcl/io/pcd_io.h>
#endif
#ifndef PCL_IO_PLY_IO_H_ 
#include <pcl/io/ply_io.h>
#endif
// boost headers
#ifndef BOOST_FILESYSTEM_OPERATIONSX_HPP 
#include <boost/filesystem/operations.hpp>
#endif

#endif

namespace pcltest
{
    // function to open PCL or binary PLY files
    pcl::PointCloud<pcl::PointXYZ>::Ptr openCloud(std::string filename);

    // function to save the point cloud to PCD format
    void saveCloud();
}

Before splitting the code into separate files, everything worked well (with the same project settings).

Edit2:

I located the source of the problem,

#include <pcl/io/ply_io.h>

causes this. For now, I got rid of everything related to PLY and everything works fine. I'll look at it later, this might be a PCL library specific issue. Still strange to me why this call causes linker error in an other file, where I don't even use PLY related functions/variables.

I had the same problem as you had. I had a surface.h and surface.cpp file, and I found out that I had to include the ply_io.h file from surface.cpp rather than surface.h and now it compiles fine. I hope that helps or makes sense! haha