probolem: link error : unresolved external symbol __thiscall ... operator ==...

//MyPolygenDlg.cpp
#include "stdafx.h"
#include "linkerror.h"
#include "linkerrorDlg.h"
#include "Polygon.h"
#include "Vertex.h"
using namespace std;
....
void ClinkerrorDlg::OnBnClickedOk()
{
    UpdateData(TRUE);

    //initialize begin
    CVertex ver1(1,1);
    CVertex ver2(2,2);
    CVertex ver3(3,4);

    vector<CVertex> arrV;
    arrV.push_back(ver1);
    arrV.push_back(ver2);
    arrV.push_back(ver3);
    //initialize end

    SortPolyPoints(arrV); //see polygon.cpp

    UpdateData(FALSE);
}

//Polygon.h
#ifndef _POLYGON_H
#define _POLYGON_H

#include "Vertex.h"
#include <vector>

void SortPolyPoints(std::vector<CVertex> &);

#endif

//Polygon.cpp
#include "stdafx.h"
#include "Vertex.h"
#include <vector>
using namespace std;

void CleanUp(vector<CVertex> & arrV)
{
    for(int i=0; i<arrV.size(); i++)
    {
        for(int j=arrV.size()-1; j>i; j--)
        {
            if(arrV[i] == arrV[j]) //link error here, under VC6.0
                arrV.erase(arrV.begin()+j); //unresolved external symbol ...
__thiscall ... operator== ...
        } //but when I went home and
rebuild all under VC2008
    } //there is no error message.
why and how can I handle this?
                                                             //thank you
very much
}

void SortPolyPoints(std::vector<CVertex> & arrVer)
{
    //1. clear same elements
    CleanUp(arrVer);
}

//Vertex.h
#ifndef _VERTEX_H
#define _VERTEX_H

class CVertex
{
public:
    CVertex(double = 0, double =0);
    CVertex(const CVertex &);
    BOOL operator == (const CVertex &) const;

    double m_dX, m_dY;
};

#endif

//Vertex.cpp
#include "stdafx.h"
#include "Vertex.h"

CVertex::CVertex(double dX, double dY)
{
    m_dX = dX;
    m_dY = dY;
}

CVertex::CVertex(const CVertex & verDest)
{
    if(this != &verDest)
    {
        m_dX = verDest.m_dX;
        m_dY = verDest.m_dY;
    }
}

BOOL CVertex::operator ==(const CVertex & verDest) const
{
    if( (m_dX == verDest.m_dX) && (m_dY == verDest.m_dY) )
        return TRUE;
    else
        return FALSE;
}