What's odd is that everything else in the section that's hidden for large screens (or vice versa) EXCEPT the form works...it's just the Marketo form that has the issue...
Technically, it's NOT working - hidden elements are still loaded in the DOM so for example a mobile user who loads your page loads all of the hidden sections as well (consuming more data and delaying the load time of your page all the same), they just do not display once they're loaded on the page b/c of the CSS. Here's a stackOverflow question that has some more info about this: https://stackoverflow.com/questions/15565586/css-displaynone-does-it-still-load-all-content-inside-t...
This is what's making the form load twice, b/c you do actually have it twice in the DOM even tho "hiding" a section appears to remove it, the way WP and these theme plugins works, it's not actually removing that content, you'd need something more sophisticated like AJAX to handle those types of situations.
It looks like you've got one form on the page now and Im not seeing the double-form issue, but there is some issue with the form overlapping the element on the right side of the page there. You can adjust your form columns, fieldwraps and inputs to a fluid width (100%) rather than a fixed width with some CSS like this:
.mktoForm .mktoFieldWrap, .mktoForm .mktoFormCol, .mktoForm input[type=text], .mktoForm input[type=url], .mktoForm input[type=email], .mktoForm input[type=tel], .mktoForm input[type=number], .mktoForm input[type=date], .mktoForm textarea.mktoField, .mktoForm select.mktoField {
width: 100% !important;
}
This'll adjust the columns, fieldwraps and inputs (except checkbox and radio) to be the same width as the column with the form. You could add this to the form's Custom CSS or somewhere in your CMS that you can add/modify your theme CSS.
Otherwise, I think you're on the right track here - it looks like you're hiding the image in the right column for mobile and showing a different one above the form instead - that might be the better way to work around the forms here by showing/hiding the content elements rather than the form.
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