Either your data is bad, or it's not structured the way you think it is. Possibly both.
To prove/disprove this hypothesis, run this query:
SELECT * from
(
SELECT count(*) as c, Supplier_Item.SKU
FROM Supplier_Item
INNER JOIN orderdetails
ON Supplier_Item.sku = orderdetails.sku
INNER JOIN Supplier
ON Supplier_item.supplierID = Supplier.SupplierID
GROUP BY Supplier_Item.SKU
) x
WHERE c > 1
ORDER BY c DESC
If this returns just a few rows, then your data is bad. If it returns lots of rows, then your data is not structured the way you think it is. (If it returns zero rows, I'm wrong.)
I'm guessing that you have orders containing the same SKU
multiple times (two separate line items, both ordering the same SKU
).