A WordPress web site client recently raised an issue about an intermittent problem with duplicate form submissions with forms generated using the Contact Form 7 plugin. We did some research and found what appears to be a solution.
I did some rooting around on the Contact Form 7 author’s site and found a fix for setting up a confirmation page that appears to solve the problem. There is a short piece of code that is added to the Additional Settings section on each Contact Form 7 editor page. You will find this section near the bottom of the editor page for each form that is created.
on_sent_ok: "location = 'http://www.mywebsite.com/request-submitted/';"
First, create a Page that contains a confirmation message. The user will be redirected to this page after submitting the form.
Second, add the code to the Additional Setting section, including a full URL to the confirmation page.
Third, do not forget to click the Save button.
The problem was reported a few weeks ago. At that time the client was receiving multiple form submissions a couple of times each day. He just reported that there have been no duplicate submissions since implementing this fix.