You need to create an acf/validate_value (https://www.advancedcustomfields.com/resources/acf-validate_value/) filter. For example you can add the filter on the name field. In the filter you need to get all of the values you are validating on and then do a query to see if there is existing posts with that value. In the past I posted an example of doing this here https://support.advancedcustomfields.com/forums/topic/accept-only-unique-values/#post-45359
