sends two, or two appear in the sent folder. The latter being a common situation with live/hotmail/outlook and gmail accounts.

Looks like just one is sent, but two appear in the sent folder. And it is an outlook account

Outlook add the mail as it goes out through their server, they also modify the name attached to the mail to that of the primary account in use. So if you call yourself say john online at outlook and J. R. Ewing in your Thunderbird. Mail will go out as John.

Right click the account in the folder pane, select settings from the menu and in copies and folders modify the save options for sent mail so you only get one. (tun off Thunderbird making a copy)