Outlooks sends multiple copies of the same email
Outlook (all versions) is known to have this quirk:
Sometimes, when sending a large email with attachment it somehow gets an error
that the email has not been sent and is still in the outbox and keeps trying to send it.
This can be caused by:
scanning your outgoing mail with an antivirus scanner: stop the scan of outgoing mail.
You may have to de-install the antivirus scanner and reinstall it without the email integration
auto send/receive is set too low:
change the "send and receive every xx minutes" to "send and receive every 10 minutes".
Anything lower than that will cause problems sending big attachments.
- the time out is set too low:
From Microsoft support, here is the fix for various versions of Outlook: