Outlook 2007 SP3 Attachments deleted and duplicated when sending multi-attachment emails

I am using Outlook 2007 SP3 running on Windows 7 Enterprise SP1. Sometimes, though not always, when I send an email with multiple attachments, one of the attachments will be deleted upon sending and replaced with a duplicate of one of the other attachments. So although all of desired files were attached and show up normally when composing the email. After it is sent, and I go into the sent folder to view the message, one of my attachments will have dissappeared and have been replaced with a duplicate of one of the other attachments. I then have to send the email over again and say, "here's the other attachment that somehow got dropped off the previous email." Why is this happening and how do I stop it?

Thanks!