Duncan enhanced the localstack/moto repository by expanding SES configuration set visibility to include SNS event destination support. He implemented an update to the SES describe_configuration_set API, enabling users to retrieve detailed SNS event destination configurations associated with SES configuration sets. Using Python, Duncan updated response templates to conditionally surface event destination details and developed comprehensive tests for creating and retrieving SNS event destinations. His work focused on backend development and cloud services, improving observability and governance for SES users. The changes strengthened validation of event routing rules and increased reliability by expanding test coverage, supporting better onboarding and troubleshooting workflows.