Generally this occurs with folk using IMAP mail with Hotmail/outlook domains and Google. What occurs is both of those providers and Thunderbird make a copy of the email to go in the sent folder, resulting in two copies being saved. As neither provider gives you the option of disabling their helpful copy, the only alternative is to disable that in Thunderbird. Account settings > copies and folders and change it to not save a copy. The draw back is Google at least do not save the BCC information on sent mail, retaining only "undisclosed recipients" so if retaining this informai0on is at all important, you probably should allow Thunderbird to continue making a copy and perhaps save in in the local folder > sent folder to reduce confusion.