I am having problems with sending the POST data to the server. If user submits the data twice accidentally then it is causing undesirables effect in the server. How to solve this issue ?

A very good article explaining ways to get rid of this Double Submit Problem is Here

Notes

Will there be a change in the url if I use sendRedirect to transfer the control within the same application(context) from one resource to another resource(E.g. from servlet to jsp)?

Yes. Definitely there will be change in the URL of the browser even if the transfer happens within the same application(or)Context.

If I block the session in the page directive like session=”false” then does the custom cookie will also be blocked.

No. Only session tracking cookie JSESSIONID will be blocked. Custom cookie will be enabled.