The most effective step in helping the auditor determine whether fraud exists after observing a double payment transaction on a supplier invoice is to perform data analytics on the supplier's information, invoiced amounts, and payments performed. Data analytics allows the auditor to systematically analyze large volumes of transactions to identify patterns, anomalies, and potential fraudulent activities. This approach is more comprehensive and effective in uncovering fraud than simply extending the audit scope or switching to a fraud investigation engagement without a thorough initial analysis.
References:
* IIA Standards: 1220.A1 - Due Professional Care
* IIA Practice Guide: Auditing for Fraud