Using unique with std::map

How can I use unique algorithm with map container?

I can't compile the following neither using STL Port, nor MSVC.8 STL
implementation:

#include <stdio.h>
#include <iostream>
#include <map>
#include <algorithm>

template <typename _PairType>
struct compare_second
{
    bool operator() (const _PairType& _left, const _PairType& _right)
const
    {
        return (_left.second == _right.second);
    }
};

typedef std::pair <int, std::string> PairType;
typedef std::map <int, std::string> MapType;

int main(int /*argc*/, char* /*argv[]*/)
{

    MapType m;

    const char* text[] =
    { "aaa"
    , "bbb"
    , "aaa"
    };

    for (int i=0; i < sizeof(text)/sizeof(text[0]);++i)
m.insert(std::make_pair(i,text[i]));

    //c:\msvs8.0\vc\include\algorithm(1150) : error C2582:
    //'operator =' function is unavailable in 'std::pair<_Ty1,_Ty2>'

    MapType::iterator end = std::unique(m.begin(), m.end(),
compare_second<PairType>());

    return 0;
}

Thanks in advance,
-Vitaly

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