Is this a bug in cryptopp for decryption twice?

[Tongle Zhou]

When I try to decrypt a packet, I find there are something wrong if the packet 
is decrypted twice. Does anyone know the reason?

Thanks!

Sample code:

    byte* convData; // store the original packet
    uint convSize; // store the size of original packet

    CryptoPP:: StreamTransformation* m_cipher = new 
CBC_Mode<DES_EDE3>::Decryption;
    CryptoPP::Filter* m_coder = new StreamTransformationFilter(*m_cipher, 
NULL, CryptoPP::StreamTransformationFilter::DEFAULT_PADDING);

    m_coder->PutMessageEnd(convData, convSize);
    m_coder->PutMessageEnd(convData, convSize);

    unsigned int outputLength = m_coder->MaxRetrievable();
    unsigned char* outbuff = (unsigned char*) cryptopp_new(sizeof(unsigned 
char)*(outputLength+1));

    //Here, the output packet is correct
    m_coder->Get((byte*)outbuff, outputLength);
    m_coder->GetNextMessage();

    //while here the output packet is wrong
    m_coder->Get((byte*)outbuff, outputLength);
    m_coder->GetNextMessage();