Given this, what I would do is….
I would create an acf/render_field action. This actions would look to find if there is already a currently set post and would output a hidden field with a true/false (1 or 0) value. The name of the field does not matter because it will not be saved, you just need to make sure it does not conflict with anything else.
Then I would create a custom javascript to fire when the true/false field is changed. If there is already a post set and the field is changed to true then a confirmation would pop up. Confirmation would leave the acf field set to true, otherwise reset the field to false.
Sorry, I don’t have any code for this example.
