While submitting the form data to a server to be handled by the Click event of Submit button, it would be a good practice to disable the button. Here is a small code sample to achieve this:
btnSubmit.OnClientClick = ClientScript.GetPostBackEventReference(btnSubmit, "") + "; this.value='Submitting...';this.disabled = true;";
Note: don't apply the tip to buttons which cause validation. Such buttons require more advanced onclick handler which checks client-side validation result.