Hi,
This is a common problem.
Cause -
The Sent Items.dbx file has reached the file size limit (2 GB).
After that OE can't copy a sent email to the SenT Items folder.
Therefore, the copy remains in the Outbox and is sent again
Solution -
First locate your store folder - Tools/Options/Maintenance/Store folder
Go to that location in Windows Explorer,
Create a new folder and drag your sent emails into it,from the Sent Items.dbx folder
close OE, delete the delete Sent Items.dbx folder,
restart OE and drag the sent items back to the recreated folder,
or leave them 'archived' in the new folder.
See
HERE and
HERE