When I send emails with attachments, I often go into my sent folder and delete the attachment (right click the attachment, then select "Delete"). Sometimes, but not always, this creates a copy of the email that has the attachment (i.e., there are now 2 identical messages in the folder, one with the attachment deleted, the other with the full attachment). Both copies are still there if I quite Thunderbird and return. This same thing happens in my Inbox as well.

This minor bug has not caused crashes or other problems, but seems strange.