DynamoDB Streams and Lambda: Duplicate Record Side-Effects in Local Development

DynamoDB Streams guarantee at-least-once delivery, so a Lambda function can receive the same stream record more than once during failover or scaling events. In a repeatable local development environment using DynamoDB Local, the stream emulator does not automatically deduplicate records, which means developers must implement idempotent logic within their functions. Options such as StartingPosition and BisectBatchOnFunctionError affect batching and error handling but do not eliminate duplicate deliveries.

Because behavior observed with DynamoDB Local may differ from the managed service, especially regarding timing and failure injection, results should be validated against a real DynamoDB instance under current runtime versions. Lambda concurrency limits and reserved concurrency settings can also affect how duplicate records are processed, potentially masking issues in low-traffic tests.

What patterns are most effective for making Lambda idempotent when consuming DynamoDB Streams in a local development workflow? How can a repeatable environment reliably simulate duplicate stream records to test side-effect prevention without diverging from managed-service behavior? Are there documented differences between DynamoDB Local and the managed service that specifically affect duplicate delivery semantics?