This problem occur if you delete a product in your shop backend and some information about its ID is not deleted.
A commond source for this error is a duplicate entry in cataloginventory_stock_item. You can search for an ID if you have one. If you want to find all duplicate product entity_ids then simply use this SQL statement:
SELECT * FROM cataloginventory_stock_item
WHERE 860 in (
SELECT product_id
FROM cataloginventory_stock_item
GROUP BY product_id
HAVING ( COUNT(product_id) > 1 )
)
If it finds duplicates, than you need to delete it.