I have two exact copies in my Sent Folder of ALL email that I send.
I actually had this problem before with 4.0. Somehow, I fixed it. I don’t remember how and can’t seem to come with the answer in search. Anyway, after the upgrade to 5.0, the problem came back.
There’s a global setting to not copy sent e-mails into sent folders. The problem with this approach is I think it affects all e-mail accounts. Not all e-mail providers automatically create “Sent” e-mails for everything that goes through their SMTP servers.
I figured out what was causing it for me… the tip-off was that it was happening to only one of my two gmail accounts (and not my non-gmail IMAP, either), and wasn’t happening on emails sent from em client.
Turns out, I had outlook set to save sent mail on gmail. Since gmail automatically saves sent mail, it was making duplicates and had been from some time.
Hence, in this case, it wasn’t an em client bug.
That might be why Sam’s case resolved itself - if he switched to em mail from outlook, it would have stopped.
Not sure why I didn’t notice this sooner, as I go in the sent folder from time to time.