I'd say it's up to you. It's not your fault to cover for their mistakes, but if you wanted to be honest, you could tell the cashier next time you are there.

At the same time it's not your responsibility to make another trip out there to let them know about their mistake. As long as you told them that you used it on a previous purchase and the cashier gave it back to you, nothing wrong with trying to use it again, they might even let you. Some cashiers don't care.

__________________