You can iterate over the array of IDs and look for the corresponding post in your results to generate output with duplicate posts:
$post_ids = array( 208, 212, 218, 208, 212, 218 );
$args = array(
'post_type' => 'custom_posttyp',
'order_by' => 'post__in',
'post__in' => $post_ids,
);
$the_posts = get_posts( $args );
foreach( $post_ids as $id ){
foreach( $the_posts as $a_post ){
if( $id == $a_post->ID ){
echo get_the_title( $a_post );
}
}
}