find-duplicate-wp-post-meta.php

<?php
global $wpdb;

$duplicates = $wpdb->get_results("SELECT PM1.meta_id, PM1.post_id, PM1.meta_key, PM1.meta_value, PM2.meta_id, PM2.post_id, PM2.meta_key, PM2.meta_value
FROM {$wpdb->postmeta} PM1, {$wpdb->postmeta} PM2
WHERE 
    PM1.meta_id > PM2.meta_id
    AND PM1.post_id = PM2.post_id   
    AND PM1.meta_value = PM2.meta_value
    AND PM1.meta_key = PM2.meta_key
--  AND PM1.meta_key = '_wp_attachment_image_alt'
ORDER BY PM1.post_id ASC LIMIT 1000 OFFSET 0");