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// WP_User_Query arguments
$args = array(
	'order'          => 'ASC',
	'orderby'        => 'user_name',
	'meta_query'     => array(
		'relation' => 'OR',
		array(
			'key'     => 'sort_order_user',
			'compare' => 'EXISTS',
		),
		array(
			'key'     => 'last_name',
			'compare' => 'EXISTS',
		),
	),
);

// The User Query
$user_query = new WP_User_Query( $args );