As you have both MSE and AVG running to scan outgoing emails then
Anti-malware Tools
The most common cause of repeated sends that can be traced back to your actual mail program is probably anti-malware tools that are trying to scan outgoing email, or otherwise interfere with the proper operation of the mail program. It's a common cause of repeated (or failed) downloads of email, and when the feature is enabled to scan sent mail, it can interfere there as well.
Depending on the type of interference, the email program responds by saying "well, that didn't work, I'll try to send it again" - and again, and again.
The solution here is pretty simple: locate that feature in your anti-malware or security suite, and turn it off. You may need to close and restart your email program, or even reboot your machine for the change to take effect.
Additionally, 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.