global variable in QT: getting ISSUES

[Girija]

hi,
I am using following code for making global variable. But I am getting the error:
:: error: duplicate symbol _m_pInstance in citybook.o and main.o
:: error: collect2: ld returned 1 exit status

My code : CityBookGlobalVariables.h

#ifndef CITYBOOKGLOBALVARIABLES_H
#define CITYBOOKGLOBALVARIABLES_H
#include <QStringList>
 class CityBookGlobalVariables
{
private:
  CityBookGlobalVariables(){};
     CityBookGlobalVariables( const CityBookGlobalVariables& _instance ){};
     static CityBookGlobalVariables* m_pInstance;
 
 
public:
 
  QStringList m_cityList;
  static CityBookGlobalVariables* instance()
  {
    if ( !m_pInstance )
      m_pInstance = new CityBookGlobalVariables;
 
    return m_pInstance;
  }
 
  void setCityList(QStringList cityList)
  {
    m_cityList = cityList;
  }
 
  QStringList getCityList()
  {
    return m_cityList;
  }
};
 
CityBookGlobalVariables* m_pInstance = 0;
#endif // CITYBOOKGLOBALVARIABLES_H

#ifndef CITYBOOKGLOBALVARIABLES_H
#define CITYBOOKGLOBALVARIABLES_H
#include <QStringList>
 class CityBookGlobalVariables
{
private:
  CityBookGlobalVariables(){};
     CityBookGlobalVariables( const CityBookGlobalVariables& _instance ){};
     static CityBookGlobalVariables* m_pInstance;


public:

  QStringList m_cityList;
  static CityBookGlobalVariables* instance()
  {
    if ( !m_pInstance )
      m_pInstance = new CityBookGlobalVariables;

    return m_pInstance;
  }

  void setCityList(QStringList cityList)
  {
    m_cityList = cityList;
  }

  QStringList getCityList()
  {
    return m_cityList;
  }
};

CityBookGlobalVariables* m_pInstance = 0;
#endif // CITYBOOKGLOBALVARIABLES_H

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please help me. Thanks in advance