How can I prevent duplicate submission emails?

I'm receiving two emails for each form submission - one that seems to be a copy of the confirmation email sent to the original use, and one where the subject line says "RE:......"  I'm sure there's a setting I'm overlooking, but how do I fix this?

I have checked your form and post a test submission. I got a autoresponder email with no duplication.

I have also checked your email log and I did not find any duplicate email sent to you.

Maybe, the user pressed twice while posting the submission.

Can you please check if you got my submission more than once in your email?