CryptEncrypt - Inconsistent behaviour with duplicate strings in Vi   

We are currently investigating becoming vista compliant and have come across 
an issue that we're not entirely certain we can fix.

We've noticed an inconsistency when trying to encrypt duplicate strings 
(this was found in testing), in that the CryptEncrypt function (in wincrypt) 
simply isn't failing with duplicate data like it's expected to.

Testing the same code in XP, the function fails as expected with the error 
code: NTE_DOUBLE_ENCRYPT -2146893806, so we were wondering if this was a bug 
that anyone else has encountered or whether it was an issue with our code. 
Any help would be much appreciated.

I'll post the code to be thorough (cutting out superfluous information):


   HCRYPTPROV hProv = 0;

   if 
(CryptAcquireContext(&hProv,NULL,MS_DEF_PROV,PROV_RSA_FULL,CRYPT_VERIFYCONTEXT) 
      || 
CryptAcquireContext(&hProv,NULL,MS_DEF_PROV,PROV_RSA_FULL,CRYPT_NEWKEYSET)) 
   {
      ASSERT(hProv);

      HCRYPTHASH hHash = 0;
      if (CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash)) 
      {
         ASSERT(hHash);

         DWORD nPasswordLen = 0;
         if (Password)
            nPasswordLen = _tcslen(Password);

         if (CryptHashData(hHash, (PBYTE)Password, nPasswordLen, 0)) 
         {
            HCRYPTKEY hKey = 0;
            if (CryptDeriveKey(hProv, CALG_RC2, hHash, 0, &hKey))
            {
               ASSERT(hKey);

               PBYTE pnt = (PBYTE)pData;
               PBYTE pResults = (PBYTE)ACQMemMalloc(10*Size);
               ASSERT(pResults);
               PBYTE pResPnt = pResults;
               long Done = 0;
               *ResSize = 0;

               DWORD CIPHBlockSize = 0;
               DWORD Length;

               if 
(!CryptGetKeyParam(hKey,KP_BLOCKLEN,(PBYTE)&CIPHBlockSize,&Length,0))
                  CIPHBlockSize = Size; // try whole lot in one call

               PBYTE pBuffer = (PBYTE)ACQMemMalloc(CIPHBlockSize + 1000);
               ASSERT(pBuffer);

               DWORD Count;
               do 
               {
                  int BlockSize = (Size-Done)> (int) CIPHBlockSize ? 
CIPHBlockSize : Size-Done;

                  if (BlockSize<=0) 
                     break;
                  memcpy(pBuffer,pnt,BlockSize);
                  Done += BlockSize;
                  Count = BlockSize;

                  if (Crypt) {
                     if (!CryptEncrypt(hKey, 0, (Size-Done)<=0, 0, pBuffer, 
&Count, 1000 + CIPHBlockSize)) {

...

This call to CryptEncrypt is where the inconsistent results are arising.

date: Tue, 10 Jun 2008 01:17:00 -0700   author:   acquire