Answer:
First, the question seems to assume that the debit is erroneous. Because, if it's intentional, then there is little need for an investigation.
So if there was an unintentional duplication of transaction, this would totally depend on what kind of debit it is. So to answer the question, we'll explore as many scenarios as possible.
Debit on a Bank Account (Business to Bank)
If it is a debit on the bank account, then the statement of account from the company side and the account activity(ies) leading up to the debits will reveal why there is a double debit. It is possible the transaction was entered twice. This can happen when there is a bad connection that creates a failed transaction message on the customer's end but still carries through with the payment instruction. In this case, the customer will think the debit or payment request didn't go through and proceed to repeat the same transaction.
Banks do execute debits on accounts for charges such as Bank Account Maintenance Charges, Credit card charges etc. Errors could also arise internally that led to the duplication of a debit instruction either due to a bug in the system.
So in this case, we'll need to access the customer's account internally whilst externally, the client will need to provide the message containing the alerts, date and time of debits etc. to help the banks further the investigation.
Business to Business
This could arise out of a "failed POS" transaction. Similar to the condition stated above, when there is a glitch with the network and the POS fails, it should print out a receipt that reads failed or declined.
This receipt will contain references to the transaction. This will aid the investigation of the debits to the account.
Step-by-step explanation: