How to disable the submit button after clicking?
It's only in dev but the Hide Submit module would do the trick. One of the features is:


  Hide (or disable) the submit button after it has been clicked


I've just installed it on a dev site and it seems to work well for node add forms; when you click the submit button it's hidden and replaced with a loading image and a 'Please wait...' message, before the form is finally submitted. I haven't tried it on any other forms though.