AWS Application Load Balancer Duplicate Requests

Solution 1:

Enable ALB access logs (they are delivered to S3) and analyse them.

More likely then not you'll find that it's actually your clients sending the request multiple times - either through user interaction (e.g. user reloads the page while waiting for the payment processing) or caused by your client side app (JS times out waiting for a response and re-sends the request). The logs will tell you.

Hope that helps :)

Solution 2:

We also have observed the same thing with a classic ELB (as opposed to your ALB). We have evidence that occasionally (a dozen of times per day in a traffic-heavy environment) 2 instances receive the exact same request, and only one of those instances is in the ELB access log.

Also, by our logs, this has been occurring for at least 3 months (and perhaps forever), but it may have gotten worse since mid-July.