Customer getting two messages causing them not to be able to order

A Customer is getting two simultaneous messages, one that the item is already placed in the cart and the other that there is no inventory for that item. But the item is never in the cart, which remains empty. What could be causing this and how do I fix it?

Using version 2.1.4.

Hello, Compact Discount

The problem requires the investigation on your server. Please contact us via Customer Help Desk and provide us with a temporary access to your server.

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