I've made a support request about this, but apparently this is not bug, but a feature instead, and I have to do a feature request to change this behavior.
Problem: a Contact can have multiple running instances in the same Campaign Automation. This causes them to receive the same emails twice, sometimes even three times.
What happens: we have a Workflow that adds a Contact to a marketing list after they've downloaded a brochure for one of our programmes. This triggers the Campaign Automation for this Contact. Contacts can download multiple brochures. If the Contact is already on the Marketing List, CRM will *not* add the Contact again, but ClickDimensions will still trigger and create a running instance for the Contact, causing multiple running instances to exist. I was told by the support staff that this is the expected behavior.
Behavior I expected: if a Contact was not actually added to the Marketing List by CRM, no running instance should be created in the Campaign Automation. Also, but correct me if I'm wrong, I can't see why you would want multiple running instances for the same Contact anyway. Sounds to me like asking for trouble...
In short, my requests: fix the "Added to list" trigger and give us, at least, the option to prevent multiple running instances for the same Contact in the same Campaign Automation.