Workflow gets triggered again if a post is unpublished and republished

## Steps to reproduce

* Create a Workflow which is triggered on a publish-related event such as "A page is published"
* Configure it to send yourself a notification
* Publish a page
* Confirm the notification appears
* Dismiss the notification
* Switch the page back to draft and then publish it again
* Observe that the notification is delivered a second time

## Expected behaviour

I would expect that publish-related events only trigger the workflow the first time the event occurs for any given post. Subsequent times the event gets triggered, it should only cause the workflow to run if it's not previously been triggered for that post.