Custom category.php paging shows the same posts

This should be

$args = array('category' => 11, 'posts_per_page' => 2, 'order_by' => 'date');

like this

$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$args = array('category' => 11, 'posts_per_page' => 2, 'order_by' => 'date', 'paged' => $paged);