In the rare case where Don & Raph's last ability resolves more than once in the same turn (probably because there were multiple combat phases), a single noncreature spell might have multiple instances of affinity for artifacts. Those abilities are cumulative. For example, if you attacked twice with Don & Raph in the same turn without casting any noncreature spells in between those attacks, the next noncreature spell you cast that turn would cost less to cast for each artifact you controlled.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as affinity for artifacts). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.