How to disable submit button once it has been clicked?

You should first submit your form and then change the value of your submit:

onClick="this.form.submit(); this.disabled=true; this.value='Sending…'; "

Probably you're submitting the form twice. Remove the this.form.submit() or add return false at the end.

you should end up with onClick="this.disabled=true; this.value='Sending…';"