You can use the Reporting Menu to group Receipts by date, amount and card number to check that there are no duplicate expense submissions - this only works for matching date, amount and card number.

To prevent employees from using the same image twice (so that the above method works), we recommend having a person review every expense submitted to make sure the date and amount on the receipt image match the date and amount on the submitted expense and the card specified in the expense entry.

Since Odoo shows the image side by side with the expense, you can access all of the expenses from a report with a smart button [via Studio] and click to open each one, with a single click to get you the next one, making this a very quick process - just seconds per receipt:

This visual check is often needed because in many countries a person must still ensure that the receipt image clearly shows the date, amount, location and nature of the expense (this can be both an auditing requirement and/or a legal requirement). 

If this solution is not adequate after implementation you can evaluate the return on investment of integrating with another expense management system.