Check for duplicate entries in the url aliases of opencart. Fix Subquery returns more than 1 row (products)

Check for duplicate entries  in the url aliases of opencart:


(same query keyword)
SELECT  `query`,`keyword`, COUNT(*) FROM  `oc_url_alias` GROUP BY  `query`, `keyword` HAVING COUNT(*) > 1


(same query) - Usually you can have errors related to the same queries
SELECT `query`, COUNT(*) FROM `oc_url_alias` GROUP BY `query` HAVING COUNT(*) > 1


(same keyword)
SELECT `keyword`, COUNT(*) FROM `oc_url_alias` GROUP BY `keyword` HAVING COUNT(*) > 1




Tip:
In most cases I fixed this error
Warning: mysqli::query(): (21000/1242): Subquery returns more than 1 row in system/library/db/mysqli.php on line 18Notice: Error: Subquery returns more than 1 row

by removing the orphaned entries/urlaliases (ex. the demo data).
Do a backup before removing anything.