Get posts with same meta value as current post

You should first get the meta value for the current post, then use it to make another query.

global $authordata, $post;
$my_meta = get_post_meta( $id, 'stashkey', true);
$authors_posts = get_posts( array(
'author' => $authordata->ID,
'post_parent' => 0,
'orderby' => 'menu_order',
'post_type' => 'page',
'posts_per_page' => -1,
'meta_query' => array(
 array(
  'key' => 'stashkey',
  'value' => $my_meta,
'compare' => '=' )
)
) );

This will fetch the posts that have equal stashkey meta as current post.