It's likely that the message isn't really sent twice, but appears in your Sent folder twice because you have instructed Thunderbird to place a copy in your Sent folder and the same setting also exist in the configuration of your email provider, e.g Hotmail, Outlook, etc. In Thunderbird check Tools > Account Settings > Copies & Folders > Place a copy in: It's likely that setting is turned on. You can try switching it off and then send a test message to yourself and see if it shows up in the sent folder anyway, if so it's because your email provider is keeping copies of your sent messages for you. It's up to you which setting you want to keep turned on, the one in Thunderbird or the one for your email provider.