uniq-field.php

<?php

/**
* This add a simple verification for selected fields if those have own value
**/

add_filter('forminator_custom_form_submit_errors', 'verification_for_unique_values_forminator_wp', 10, 3);

function verification_for_unique_values_forminator_wp($submit_errors, $form_id, $field_data_array) {

	  $forms = ['2071', '1020'];
	  $fields = ['name-1', 'email-1', 'phone-1'];
	  $unique = [];

	  // Skip if is not the correct form
	if( !in_array( $form_id, $forms ) ){
	    return;
	}

	//Make sure the received value are lowecase
  	foreach($fields as $field){
		$unique[] = strtolower( $_POST[$field]  );
	}

	// verification for duplicated values
	if( count($unique) !== count(array_flip($unique))) {
		$submit_errors[][ 'name-1'] = __( 'Please, verify any duplicated entry' );
  	}

	  // Always return $submit_errors
	  return $submit_errors;


}