Are you confused when scheduled Lambdas execute twice, SNS messages trigger an invocation three times, your handmade S3 inventory is out of sync because events occurred twice? Bad news: Sooner or later, your Lambda function executes multiple times. You have to prepare yourself! The reasons are system failures, retries on errors, and event sources that guarantee at-least-once delivery (e.g., CloudWatch Events, SNS, …). In this talk, you learn why Lambda functions might execute twice and what you can do about it focusing on Lambda, DynamoDB, and Kinesis Data Streams.