Answer
This is covered by IFS Connect's own logging and debugging configuration, not a separate Biz-API-specific log — the relevant log files live in the application server's own IFS Connect log directory, and IFS's Logging and Debugging IFS Connect documentation explains both where to find them and how to raise the logging level so that successful (not just failed) calls are actually recorded. Raising the log level before trying to reproduce or capture the reported duplicate-message behavior is the way to get visibility into every inbound Biz API call, not just the ones that error out.