Hello Sriram, 

Thanks for posting your query in PSC :)

In case,  you are calling finish assignment, then refer the post. Also, it suggests on Close Harness.

Regards,

Rincy

I think if you set the dynamic container to single document it always closes the earlier document while creating new document. So creating new case would close the earlier case.

-Saikat

Or you can try calling this script to close all the previous thread.

<script>
function closeDocuments(event){
pega.desktop.closeAllDocuments(event)
};
</script>

-Saikat