There are a number of historical planetary missions that have produced 2 flying instances. Examples include Voyager, Viking, MERS, some of the Mariners missions. Other missions have only one instance: Mars Observer, MRO, Magellan, Cassini and so on.
What is the cost difference between producing a mission with one instance vs. a mission with two identical instances. I'm looking for specific citations to clarify and answer this point.
This question presumes that the production of the second instance occurs at the same time as the first instance. That is to say in parallel, not produced at a later time.