Forms being resubmitted days after first submission

We have an order form that clients fill out to send us print requests. Over the last couple of weeks we have a number of submissions that have been resubmitted days later. It looks to us like a new order, then we recognize it as a duplicate order of an order placed days earlier, and in one case weeks earlier. It appears there is a ghost in the machine.....

Most likely your user submitted the form and did not close thank you page. The page was then reloaded or back button was clicked which triggered another submission. 

Your form is embedded with full source code. You can re-embed it with iFrame code and add Captcha field to your form to prevent duplicate submissions if thank you page is not closed. 

Getting the form iFrame code

How to Add a Captcha Field

You can also prevent duplicate submissions on a form by using Unique Submissions or Unique Question feature:

How to Set Unique Submissions on a Form

How to set Form Limits Based on a Unique Question

Hope this will help. Let know if you need further assistance.