Review the History for the customer in the CRM to verify whether they did already cancel.

If the customer was billed by mistake, let the customer know you will get back to them and escalate to your manager.

If there is no record of the cancellation in the CRM, try to move the conversation along.

Talk Track: Odd, I'm not seeing a record of that. Do you have a copy of the cancellation confirmation email that we send? No problem, I can confirm that your account is now cancelled and you will receive no further shipments.