I wanted to share my recent experience with this very same problem. No matter what I tried, any front-end form I created with acf_form was creating duplicates. I’ve worked with ACF Pro for a long time and had never run into this problem on other projects.
Based on the advice here, I tried switching themes, and the problem stopped. So I knew something was up with the custom theme I’d built. I finally found the problem by creating a blank new theme, then rebuilding the theme file-by-file to find the exact template file that caused the error.
FINALLY I discovered that the problem started when I added footer.php. After investigating that file, I noticed that ‘<?php wp_footer(); ?>’ was missing before the closing body tag. Adding that immediately fixed the problem.
Missing the call to wp_footer() seems to have been the culprit in my case. Hopefully this helps someone else out there.
