This is how I fixed the problem:
I used the init hook to load in acf_form_head() like so:
function form_head() {
if (!is_admin()) {
acf_form_head();
}
}
add_action('init', 'form_head');
At first I used the init hook without the is_admin check, this caused that acf_form_head to load twice/post twice.
The solution is to use the init hook with is_admin() to prevent it loading twice.
