The reason codes for chargebacks related to duplicate processing are:
Visa: 82 - Duplicate Processing
Mastercard: 4834 - Duplicate Processing
Discover: DP - Duplicate Processing
Amex: P08 - Duplicate Charge
For these kinds of chargebacks, the cardholder believes that they were incorrectly charged twice for the same transaction. The chargeback notification itself contains information on both charges:
From this information, users can use the Find Checks utility within the Toast Portal to find the affected checks, which can be used to find the signed receipts on hand at the location or by themselves in the event that signed receipts are not available.
Using the Find Checks tool to find the affected checks is documented here, under the section "Using the Find Checks Tool".