Hello Simon,
This duplicate email issue I believe is connected to having the form trigger on. ( triggers are run by Google)
You could try taking the trigger off and then running manually when you want to send out the emails or frequently, creating a completely new spreadsheet solves the problem. (I know this can be a lot of work)
I have been following the Google Issue Tracker and many people have reported this to Google ( it is not only happening in Form Mule) I would suggest that you follow the Issue Tracker and also let Google know that you are seeing this error. Notifying them that is still occurring is helpful. Google has acknowledged the issue ( comment #19) and say they are working on a fix.
Please keep us posted on if you see any improvements! Thanks.