When adding new items to a stock list, a separate line is added even though the same grocery was already on stock. This is by design, as the other item has previously been added and thus has a separate stock date.
However, if the stock dates are really close, you could open one item and increase it's quantity and remove the other identical items. Doing so will make the stock list shorter and easer to get an overview of. Also when removing from stock, the app can just decrement the item quantity.
Note: Use barcodes whenever possible to add or remove stock items - this makes it much faster and less error prone to bring the list up-to-date.