This was absolutely, 100% fixed in version 2.2.1. We spent around 8 hours thoroughly debugging the issue, finding the cause and resolving it to prevent this double post bug.
Your issue lies with WordPress:
I think I found what is causing this behavior. I have missed schedule plugin installed that goes through all posts regularly to see if anything is missed schedule. I have to use this because for some reason my WP doesn't publish posts on scheduled time.
You shouldn't need to rely on a third party plugin to publish your posts - WordPress is more than capable of handling scheduled posts itself.
We wouldn't provide support for this third party configuration. Happy to support if you get scheduled posts working on WordPress without the use of a third party plugin and still get the double post bug behaviour.