Prevent duplicate CRED submissions where a custom field value has already been used

The below code will check if any existing posts already use the same value for a custom field submitted in a CRED form.

/**
 * CRED custom validation to prevent duplicates
 */
add_filter( 'cred_form_validate', 'prevent_duplicate_submissions' );
function prevent_duplicate_submissions( $error_fields, $form_data ) {

	$post_type = 'cpt-slug';  // Edit
	$unique_field = 'wpcf-' . 'field-slug';  // Edit
	$form_ids = array( 101, 115 );  // Edit IDs of CRED forms

	list( $fields,$errors ) = $error_fields;

	if ( in_array($form_data['id'], $form_ids ) ) {

		// Get existing posts with unique field
		$args = array(
			'post_type'		=>	$post_type,
			'post_status'	=>	'publish',
			'numberposts'	=>	-1,
			'meta_key'		=>	$unique_field,
			'meta_value'	=>	$fields[ $unique_field ]
		);

		$matching = get_posts( $args );

		if ( !empty( $matching ) ) {

			$errors[ $unique_field ] = 'Value already used';
		}
	}

	return array($fields,$errors);
}