Showing processing indicator in contact us form

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I have a contact us page in my website using ASP.NEThttp://sosdeepcleaning.com/contactus.aspxWhen clients fill up the form and click submit, it doesn't show any sign that the page is processing the form, so some of them click on the submit button twice. How can I prevent them to click it twice? Dialog box? "waiting" bar? Alert?Any easy solution I can add before Response.Redirect?Thanks