Jack, I do not know if they are related or not. I do not have any of the MS-Exchange lines through any of my headers. That said, it does look like the headers may hold some of the keys since they are significantly different; however, I do not have the time to fully parse and sanitize them so that I would feel comfortable posting on an open forum. Also, I cannot tell if it is the Mailster component or the CMandrill component is causing the issue. But regarding the occasional aspect, I have noticed that some file attachments (such as PDFs) do not generate duplicate emails while others (such as CSV and XLSX) will. I have not run through all the various file types to determine which ones may or may not be an issue.
Daniel, I can provide more information on what causes the issue and am ready to grant you access to the site and send detailed instructions on how to replicate. Just let me know how best to send you this information.