Pobox might be one of those services, like gmail and outlook.com, that automatically copy sent messages to the Sent folder, in which case unchecking 'Place a copy in' in the Copies & Folders section of Tools/Account Settings should eliminate the duplicates and the error. Is that the case?

In Thunderbird if you don't check "Put A Copy In The Sent Folder" it doesn't put one there. I have sent several with that box unchecked and POBOX doesn't put a copy anywhere either. So It would appear unchecking that function did not fix my issue. Thanks for your help.

If it only happens with messages with attachments, I'd look at any other program that might be scanning outgoing mail, such as security/AV. It's generally recommended to exclude the TB profile folder from AV scanning, leaving the real-time background scan in place. What is your AV, and is it set to act on TB?