The Campaign Clickthroughs metric is a renamed (branded) eVarN Instances metric. it returns the number of times the value was set to the variable. If the same value was set twice, the metric would return two.
If the implementation leverages another Clickthroughs metric that is a custom event with the serialization setting "once per visit", then this event will return one for the same scenario. Similar is true if the metric is built as a derived metric on a visit.