First thing is that
acf/pre_saved_post will only work with an acf_form() on the front end of the site and will not work in the admin so
is_admin() is doing nothing for you. You’d need to do something completely different than using
acf/pre_saved_post if you want this to happen in the admin as well and I don’t know what that is.
Second issue is that in order to duplicate the post you need to omit the ‘ID’ value. $post_id already exists. According to the WP docs, this has the effect of updating the existing post rather than inserting the new post, so you are not changing the post ID.
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