1) Use an acf/validate_value filter on the date field to return an error if the date is already used https://www.advancedcustomfields.com/resources/acf-validate_value/
2) Implement this on the date field in combination with the ACF JS API (I don’t know how to do this, just something to think about) https://dzone.com/articles/disable-dates-in-datepicker, https://www.advancedcustomfields.com/resources/javascript-api/
