Add Featured image column into wordpress admin on pages AND posts

The reason you’re getting an error is because you’ve got multiple functions with the same name. You have two functions called add_img_column() and two called manage_img_column(). You can’t have two functions with the same name in PHP.

You don’t even need two functions though. You can hook the same function into multiple hooks. So this is all you need:

add_filter('manage_posts_columns', 'add_img_column');
add_filter('manage_pages_columns', 'add_img_column');
add_filter('manage_posts_custom_column', 'manage_img_column', 10, 2);
add_filter('manage_pages_custom_column', 'manage_img_column', 10, 2);

function add_img_column($columns) {
  $columns = array_slice($columns, 0, 1, true) + array("links" => "Image") + array_slice($columns, 1, count($columns) - 1, true);
  return $columns;
}

function manage_img_column($column_name, $post_id) {
 if( $column_name == 'links' ) {
  echo get_the_post_thumbnail($post_id, 'thumbnail');
 }
 return $column_name;
}