Apparently, Trojitá stores mails to the "sent" folder before sending them. When the sending fails (e. g. by typing the wrong password) and it's repeated and then the mail is sent successfully, it's stored twice to the sent folder. Reproducible: Always Steps to Reproduce: Try to send a mail. Type the wrong password. Send it again typing the correct password. The mail appears twice in the "sent" folder.
*** This bug has been marked as a duplicate of bug 354394 ***