Declaration conflicts in win32 static library in visual studio 2017

class CString
{
    // Members
protected:
    char *m_str;

    // Constructor/Destructor
public:
    CString() 
    { 
        m_str = NULL;
    }
    CString(char *szInputSource) 
    { 
        m_str = NULL;
        if (szInputSource)
            Copy(szInputSource);
    }
    CString(const CString &szInputSource) 
    { 
        m_str = NULL;
        Copy(szInputSource.GetData());
    }
    ~CString()
    {
        Empty();
    }

// Methods
public:
    void operator +=(CString &szAdditionalString)
    {
        Append(szAdditionalString);
    }

    void operator +=(char *szAdditionalString)
    {
        Append(szAdditionalString);
    }

    void operator +=(char sAdditionalChar)
    {
        Append(sAdditionalChar);
    }

    CString operator +(CString &szAdditionalString)
    {
        CString szTemp = *this;
        szTemp.Append(szAdditionalString);
        return szTemp;
    }

    CString operator +(char *szAdditionalString)
    {
        CString szTemp = *this;
        szTemp.Append(szAdditionalString);
        return szTemp;
    }

    CString operator +(char sAdditionalChar)
    {
        CString szTemp = *this;
        szTemp.Append(sAdditionalChar);
        return szTemp;
    }

// binary + operator (global, with const parameters)
    friend CString operator+(const CString &sz1, const CString &sz2)
    {
        CString s = sz1;
        s += (CString&)sz2;
        return s;
    }

    friend CString operator+(CString &sz1, const char sz2)
    {
        CString s = sz1;
        s += (char)sz2;
        return s;
    }

    friend CString operator+(const char *sz1, CString &sz2)
    {
        CString s = (char*)sz1;
        s += sz2;
        return s;
    }