First of, this is a really good plugin and I've decided to give it a go because Contact Form 7 file attachment just didn't want to work. Therefore, kudos to cForms developer(s)! :D
Anyway, I'm not sure if this is a bug but I found it when trying to get file attachments to work. This is I simulated the bug.
1. Create form with file upload. (doh)
2. Submit form successfully. Don't leave or close this page.
3. Check email to ensure attachment received.
4. Return to submission page, hit Refresh (I did a Ctrl+F5).
5. Same submission and message is sent to my email again.
If this is indeed a bug, I do hope the developers have a solution to this. :)
P.S.: Sorry if it's a known issue. Couldn't find it when I searched and I'm a n00b here. LOL!