There are only a couple of reason that I can think of for duplicate posts being created. One is that there is another pre_save_posts filter running. The second is that there is another plugin that’s creating a new post.
Have you tried disabling all other plugins on the site? Do you know if there are any other filters running when the form is submitted?
One last thing that may cause it, but it’s way out there, is calling acf_form_head() twice.
