409 error while submitting a response

A user is trying to submit response through ODK Enketo (ODK Central) for a form and he is getting a 409 error, but with the same form the user has submitted responses a few hours back.

A 409 means that the server already has a submission with the ID of the submission that is being sent. Did the user notice anything unusual when submitting at any time? Is there something distinctive about the submission they're attempting to send that would let you quickly confirm whether the same data has already been sent? Either somehow the same id was generated for two independent submissions (should be very rare) or something didn't go quite right about the submission the first time it was attempted and it's now an unexpected re-attempt.

Can you tell us more about the web form and how it has been distributed to the user? Is it a Public Access Link or is the user a Central Web User? Did you add /x/ to make it offlineable or is it a standard online-only form?