How to create a tool header file without symbol duplication error in VC?

I am trying to write a tool header file to be used for many projects.
However, I am always getting the symbol duplication error.

I have tried to use the following trick to allow my code to be included
once:

#ifndef _HOOKENGINE_H
#define _HOOKENGINE_H
.....
[code]
#endif

However, I am still getting the multiple definition errors:

Error 1 error LNK2005: "void __cdecl MyPrintf(wchar_t const *,...)"
(?MyPrintf@@YAXPB_WZZ) already defined in HookEngine.obj dllmain.obj
Error 2 error LNK2005: "void __cdecl Test(void)" (?Test@@YAXXZ) already
defined in HookEngine.obj dllmain.obj
Error 3 fatal error LNK1169: one or more multiply defined symbols found
D:\Program\Win32\HookEngine\Debug\HookEngine.dll 1

Can anyone tell me the reason? Thanks!

extern void __cdecl MyPrintf(LPCTSTR lpszFormat, ...);

#ifndef _HOOKENGINE_H
#define _HOOKENGINE_H

#define MAX_PRINT_STRING 1024
//#define MSG_BOX_PRINT
#ifdef _DEBUG
#define MSG_DEBUG_PRINT
#endif

#define MSG_DEBUG_PRINT

#ifdef MSG_FILE_PRINT
TCHAR szFilePrint[MAX_PATH] = TEXT("c:\\out.txt");
BOOL bZeroFile = FALSE;
#endif

void __cdecl MyPrintf(LPCTSTR lpszFormat, ...)
{
 TCHAR szOutput[MAX_PRINT_STRING]; // max printable string length
 va_list v1;
 va_start(v1, lpszFormat);
 //DebugBreak();
 StringCchVPrintf(szOutput, MAX_PRINT_STRING, lpszFormat, v1);
#ifdef MSG_DEBUG_PRINT
 OutputDebugString(szOutput);
#endif
#ifdef MSG_CONSOLE_PRINT
 _tprintf(TEXT("%s"), szOutput);
#endif
#ifdef MSG_BOX_PRINT
 MessageBox(NULL, szOutput, TEXT("MyPrintf Output"), MB_OK);
#endif
#ifdef MSG_FILE_PRINT
 {
  static HANDLE hFile = NULL;
  DWORD dwNumWritten;
  if (hFile == NULL)
  {
   hFile = CreateFile(szFilePrint, GENERIC_WRITE, FILE_SHARE_READ |
FILE_SHARE_WRITE, NULL,
    bZeroFile ? CREATE_ALWAYS : OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
   //if (hFile == INVALID_HANDLE_VALUE)
   // MyPrintf("CreateFile output log file %s failed with %s\r\n",
szFilePrint, FormatError(GetLastError()));
  }
  if (hFile != INVALID_HANDLE_VALUE)
  {
   OVERLAPPED ol = {0};
   LockFileEx(hFile, LOCKFILE_EXCLUSIVE_LOCK, 0, 1, 0, &ol);
   SetFilePointer(hFile, 0, 0, FILE_END);
   WriteFile(hFile, szOutput, lstrlen(szOutput)*sizeof(TCHAR),
&dwNumWritten, NULL);
   UnlockFileEx(hFile, 0, 1, 0, &ol);
  }
 }
#endif
}

#endif