Query does not filter duplicate _sku numbers

I haven’t tested this, but this should give you the count of skus for each post_id:

SELECT post_id, COUNT(`post_id`) AS sku_count FROM `wp_postmeta`
WHERE `meta_key` LIKE '%sku%'
GROUP BY `post_id`

To have it return only rows where the count is greater than one is trickier. Off the top of my head I’d probably use the above as a subquery:

SELECT * from (< insert the above query >) AS counts where counts.sku_count > 1

Might need some tweaking.

If you can avoid using LIKE, or at least get rid to one of the % wildcards, you will improve performance