How does one stop processing of a second click?

I face this issue both in a regular form and a stateless form. The processing of actions for both necessarily take time on the server side, and "Loading" alert pops up sometimes, but frequently it doesn't. The user is therefore tempted to push Submit again, which leads to a duplication of data creation.

I found the "cancel submit" option for forms, based on the data having already been created; however, a user pushing the button a second time quickly will beat this creation process and a second set is created anyway.

In the stateless form, cancel submit isn't valid.

Is there a way to prevent actions from further clicks? Any other suggestions that can help?