Simultaneous add to cart causes checkout problem

My client sells single quantity products and backorders are disallowed. So, in theory a product should only be allowed to be added once to a user’s cart. I was able to trigger a simultaneous add to cart for one of the products by two users. What happened next was a message indicating there was a problem with the product and the user should return to the previous screen. In both user sessions I was unable to view the cart to attempt to remove it. I had to restart one of the user devices to clear the product from the cart.

A device restart is a radical solution in my opinion. What is the best way to resolve this?

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