Sounds like an over eager clicker. The next release will include some Javascript to disable the submit button and hopefully avoid excessive clicks/submissions/emails.
Thank you for replying. Previously I don’t have this kind of issue, it appears after I adding new field.

We’re experiencing this as well. Is it simply the client clicking the submit button twice before the thank you is displayed?
Is there any update to this feature? It is a frequent problem for my sites as well, and the comment about the next release was 7 months ago. When can we expect the update? If it isn’t soon is there any workaround you would suggest?