Hi,
Try disabling the button on first click or change the text from submit to submitting...with javascript
<input type="submit" id="btnSubmit" value="Submit"
onclick="this.disabled=true;this.value='Submitting...'; this.form.submit();">
For the server side button,
OnClientClick call a java function which does this work of disabling and changing text.And onClick of the asp button, call the asp method you want.
hope this helps.