The most helpful technique for an IS auditor to determine whether duplicate vendor payments exist is Computer-assisted technique, which is discussed in ISACA's CISA Study Manual. According to the manual, computer-assisted techniques use computerized techniques to identify potential errors and irregularities in a system. These techniques can be used to detect errors such as duplicate payments, which can be easily missed in manual systems. In addition, these techniques can be used to analyze data to detect patterns or trends that may indicate fraud or misappropriation of funds.