In real life copying a key isn't something overly difficult, but let assume that it would be done by npc for a significant price like 500caps. You would have to come to npc with a key and give it to him by saying its number, then paying 500 caps and you would get a copy.
Having back-up keys would probably make players more willing to use locks. Also an idea of collective locker to which a few people with keys have acces would be possible.