Outlook Addin duplicated attachment - outlook

Question

I have an Outlook addin written in unmanaged C++ which uses Outlook Object Model API.
The code copies Microsoft Office files (doc, docx, xls, xlsx, etc...) and PDF attachments to the temporary folder. Then the code manipulates those files, and after that, it deletes the original attachment files and attaches the processed files.
Recently one of our customers reported duplicate attachment issue. I put some additional diagnostics, and here is what I see:
Here is the pseudo code of the diagnostic function:
bool COutlookAttachmentHelper::AreThereDuplicatesOnInput(long count)
{
std::vector<std::wstring> vFileNames;
for(int i = count; i > 0; i--)
{
//get attachment filename
.....
// try to find it in the vector defined above
std::vector<std::wstring>::iterator findIter = std::find(vFileNames.begin(), vFileNames.end(), strFileName);
// if not found push it into the vector and return false
// if found, write the filename to the log file, pop up the message
// box, return true (cancel email).
}
}
If the duplicates aren't found, then the code executes another loop to copy the files of interest to the temporary folder
the for loop is constructed the same way:
Here is what I see in my diagnostic log file:
Searching for the duplicates
Some Word Document.DOC
image003.gif
image002.png
image001.png
End of Searching for the duplicates
// Below is the output from the second loop
// I'm filtering out non Office and non PDF files
Some Word Document.DOC
Some Word Document.DOC
We can't reproduce the issue in our labs and none of the other customers report it.
Anyone have a clue?
Regards,
Michael