Instead of having a key for each gate that can be copied, make two types of keys: master, and minor keys. Both types will be able to open the gate, but only the master keys can be copied (copied as either another master, or a minor). The key you get when you create the gate will be a master, of course.
In addition to this, make it so that the master key can also be used to re-key the gate. Doing this would make all other previous keys to that gate (including other masters?) no longer work. A helpful feature would be to label keys that no longer work, when you try to use it on its gate.
Intended effects: make the effects of losing a key less drastic. Now gates can be re-secured when a key is stolen or lost without having to destroy the wall. This is an alternative to Jackard's idea (viewtopic.php?f=5&t=5309), which would totally make it impossible to steal keys.