BlueMail leaves the filing of sent mail to your email provider, which is exactly what modern providers expect. Gmail, Outlook, Yahoo and most IMAP hosts save a copy to your Sent folder themselves the moment they accept the message, so the great majority of accounts see one clean copy and nothing else to configure.
If You Do See Two Copies
A duplicate means two things saved a copy of the same message. Because the copy is written on the server, that is where the setting lives, and your provider's webmail is the place to adjust it.
Sign in to your account in a browser and look through the mail settings for an option covering saved or sent messages. Providers name it differently, and it is usually worded along the lines of saving a copy of outgoing mail to the Sent folder. Turning off the extra copy leaves you with the single one your provider files automatically.
If you are not sure which setting applies to your account, your provider's own help pages are the fastest route, since only they know how their servers file sent mail.
Tidying Up What Is Already There
Existing duplicates are ordinary messages, so you can clear them the usual way. Select them in your Sent folder and delete, or clear a run of them at once with multi-select. See How to do operations on multiple mails, in bulk? for the quickest way to do that.
