Why is Outlook Express sending multiple copies of my email?

The situation here is that your email program, Outlook Express in this case, is repeatedly sending a message over and over.

There are two common causes.

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.

Outlook Express Mailbox Limits

Outlook Express apparently has a 2 gigabyte file size limit for its mailboxes.

Normally, that doesn't affect most people, but I've definitely seen reports of it doing exactly what you've described when the Sent Items folder approaches 2 gigabytes. Smaller emails might make it out for a while, but a larger attachment might be enough to exceed that size limit, and thus cause the problem.

My recommendation is to create a new folder - call it what you will; perhaps something like "SentArchive", and move all the messages in your Sent Items folder to the Sent Archive folder. That should a) retain all your sent messages should you need to refer to them, and b) empty the Sent Items folder so that this problem doesn't happen again.