PRODUCT ADMIN - Subquery returns more than 1 row

Hello,

I have upgraded from 2.0.0.1 to 2.3.0.2 mainly for a new design.

After exporting and importing all products and categories from one to the other everything seems to work fine on front end, but on some products in the admin I get the following error - "Warning: mysqli::query(): (21000/1242): Subquery returns more than 1 row in *filepath*/system/library/db/mysqli.php on line 18" - after investigating, it seems it's a duplicate table / table entry inside the database.

I am very new to databases and not sure how to go about finding and removing the duplicate entry(s). Line 18 in the mysqli file reads - "$query = $this->connection->query($sql);" - If somebody could show me how to write "LIMIT1" or similar that might work too.

Any help would be greatly appreciated.

Thank you.

any help/ideas at all would be great.