DuplicateSessionDetected issue : We are getting duplicate session exception when we try to stop our second webserver where the request is getting processed ?

We are trying to do failover testing of the Application Server as well as Webserver.

Following are the steps that we perform

1. Have both the webserver in running state

2. Stop the first webserver that will have the request route to second webserver. Now the request will be routed from second webserver.

3. Start the first webserver and then stop the second webserver. Here is where we get Duplicate Session exception as follows.

It would be great if someone can guide me to resolve the issue.