Prevent double submits
Preventing it on the server side is not trivial - a second call may hit a different instance. So you need to deal with sessions. The code will get complex quickly.
I would recommend disabling the button before a call and reenabling it upon a response.
I am using GAE for an app that has various submit href buttons, and use javascript to submit.
I am having a real tough time trying to figure out how to prevent multiple submits or doubl-clicking.  I have tried various methods to disable or remove the href with javascript.
But I am thinking if there is maybe a method to prevent this in the backend.
What methods would you recommend I use?