I encountered a problem where an action item in a workflow would go to multiple participants. Each participant gets an email requesting action. If one participant actions the item (approves/rejects) and another participant opens the same item after it has been actioned an error occurs.

Then as a sub rule add the Error condition.

Then add an advanced condition

Then you can choose to show a custom message to the user or navigate to another form. Maybe a custom error form. In this case I simply put a custom message and closed the browser..