Hi,
If it says that two downloads were made, it means that the person must have clicked twice on the download link. The person must have done it without realising what she was doing.
There is nothing in the interface to reset the download count for an order. You'll have to run the MySQL query below through your PHPMyAdmin:
DELETE FROM #__hikashop_download WHERE order_id = XXX;
where #__ is to be replaced with your table prefix (that you can find in the Joomla configuration) and XXX to be replaced with the ID of the order.