Duplicate testing is an analytical technique used to detect fraudulent payments, errors, or inefficiencies by identifying repeated transactions within financial records. In this case, an internal auditor would use duplicate testing to ensure that employees are not receiving fraudulent invoice payments by verifying that no invoice has been paid multiple times.

Detecting Duplicate Payments: Fraudulent employees may submit the same invoice multiple times with slight modifications to avoid detection. Duplicate testing helps find identical or similar transactions.

Identifying Unusual Patterns: By analyzing payment records, auditors can detect repeat payments to the same vendor, same invoice number, or similar amounts within a short time frame.

Aligns with Fraud Prevention Practices: As per IIA Standard 2120 - Risk Management, internal auditors must identify and assess fraud risks, including duplicate invoice payments.

Supports Data Analytics in Auditing: IIA GTAG (Global Technology Audit Guide) 16 - Data Analysis Techniques recommends using duplicate testing to identify fraud, control weaknesses, and errors in financial transactions.

A. Perform gap testing: Gap testing is used to identify missing data or transactions in a sequence (e.g., missing invoice numbers), but it does not specifically target duplicate or fraudulent payments.

B. Join different data sources: This method is useful for cross-checking information across multiple databases, but it is not directly related to identifying duplicate invoice payments.

D. Calculate statistical parameters: Statistical analysis provides summary insights about data (e.g., mean, median), but it does not specifically detect duplicate payments.

IIA Standard 2120 - Risk Management: Internal auditors must evaluate fraud risks, including duplicate payments.

IIA Standard 1220 - Due Professional Care: Requires auditors to apply appropriate data analytics techniques.

IIA GTAG 16 - Data Analysis Techniques: Recommends duplicate testing as an effective fraud detection method.

Key Reasons Why Option C is Correct:Why Other Options Are Incorrect:IIA References:Thus, the correct answer is C. Perform duplicate testing.