Thanks for starting a thread Vijay. :-)
Are you deploying through *-object.xml? If so, this is a known issue and a limitation of JBoss Portal. I would have to search for jira.
When you deploy your application using object.xml, it creates an entry in DB but when you undeploy, it does not remove the entry from DB. I believe if you create instances and add to pages using portal admin ui, then this should not be the case.
Please let me know if anyone has faced the same problem and got any solution for the same
there's already a JIRA-ticket regarding this issue https://jira.jboss.org/jira/browse/JBPORTAL-1894.
A workaround could be to delete the instance first via portal admin ui and then undeploy the war-file.