VC编译出现一个错误

[xfxdxcy]

我在编译一个项目的代码时。出现这样的错误：
D:\Program Files\VC++\VC98\MFC\INCLUDE\afx.h(432) : error C2535: 'const class CString &__thiscall CString::operator +=(char)' : member function already defined or declared
D:\Program Files\VC++\VC98\MFC\INCLUDE\afx.h(429) : see declaration of '+='
我百度了很久也没找到原因，请高手们帮忙解决下
部分代码：
// concatenate from another CString
const CString& operator+=(const CString& string);

// concatenate a single character
const CString& operator+=(TCHAR ch);
#ifdef _UNICODE
// concatenate an ANSI character after converting it to TCHARconst CString& operator+=(char ch);
const CString& operator+=(char ch);
#endif
// concatenate a UNICODE character after converting it to TCHAR
const CString& operator+=(LPCTSTR lpsz);
红色部分是出错的地方

[Athenacle_]

#ifndef _UNICODE
// concatenate a single character
const CString& operator+=(TCHAR ch);
#else
// concatenate an ANSI character after converting it to TCHAR const CString& operator+=(char ch);
const CString& operator+=(char ch);
#endif

[氰客]

#ifdef _UNICODE
// concatenate a single character
const CString& operator+=(TCHAR ch);
#else
// concatenate an ANSI character after converting it to TCHARconst CString& operator+=(char ch);
const CString& operator+=(char ch);
#endif