How to submit form only once after multiple clicking on submit?

Disabling the button is one solution, but potentially poses a problem for keyboard users who just hit enter to submit a form. In this scenario, the button won't be disabled. The sure-fire method would be to handle the onsubmit event like so:

(function () {
    var allowSubmit = true;
    frm.onsubmit = function () {
       if (allowSubmit)
           allowSubmit = false;
       else 
           return false;
    }
})();

(well, as sure-fire as you can get with JS enabled anyway). You could disabled the button as a visual confirmation to the end user that the form can only be submit once too.

<input type="submit" onclick="this.disabled = true" value="Save"/>