I am doing a migration from very old to current wordpress/woocommerce and while investigating the data I found out that there are more than one row for a products _stock meta key. The same for other meta_keys.

I found an old discussion here https://core.trac.wordpress.org/ticket/10905

Which relations (tables) utilize multiple meta values in the meta table?

Which relations break if I delete all but one of the values?

Can I safely force a UNIQUE index?

ALTER TABLE `wp_postmeta` ADD UNIQUE (
  `post_id` ,
  `meta_key` ,
  `meta_value`
);