Outlook sending multiple copies of email

I’m having a problem with Outlook sending multiple copies of some emails – usually those with attachments. It doesn’t happen with evey email with attachments, just occasionally. It will send it 20-30 times but keeps the email in my Outbox saying that it hasn’t been sent (although sometimes it says it’s been sent but stays in the Outbox).
How can I solve this?

This can happen when you are using a virus scanner that integrates with Outlook and therefore does an outgoing mail scan. In those cases you can disable the outgoing mail scan or even uninstall the entire integration and see if the issue goes away. For steps on how to properly disable the integration see the documentation for your virus scanner.

For more background information see "Duplicate E-mails" and "Disable virus scanner integration?"