Hi Joshua,
A possible reason is that we execute another Marketing form API at onsubmit event of form.
<form id="contactForm" onsubmit="submitForm()">
function submitForm() {
var form = document.getElementById("contactForm");
MsCrmMkt.MsCrmFormLoader.sendFormCaptureToCrm(form);
}
Then it will create two submissions, but actually they are not duplicate, because the second submission will be used to update matching contact.
To confirm this, we can add "Contact matching result" field to Marketing form submission form manually to check whether the field value of two submissions are different.
We can find that submission 1 and 4 works as expected, because I removed the sendFormCaptureToCrm function.

Comparison between record 2 and 3.


Regards,
Clofly