We've received several calls recently from folks inquiring about having copies made of keys marked "Do Not Duplicate". In general, unless we have installed or serviced the lock, we will not make copies of keys that are so marked. We respect the apparent intent to control the number of duplicate keys.
That being said, there are places that will duplicate such keys, apparently without any respect for the original intent. With few exceptions (some government and post office keys), there is nothing illegal about duplicating a key marked "Do Not Duplicate".
If you want to be certain that no unauthorized duplicates are made of a key, that is when you need to turn to what are generally referred to as "high security" locks. That generally means that the manufacturer has a restricted, patented keyway. Some of these locks will only be sold to certain locksmiths, or a single locksmith, in a geographical area. Bear in mind that, if you invest in these types of locks, you are probably limiting yourself to that same locksmith for all future service on those locks. We want our customers to come back because they want to, not because they have no choice.
Another recent call we received was from a business that had invested in a high-security lock just five years ago, but the original locksmith would no longer service the lock - its particular patent had run out, and they would no longer be serviced. Replacement was the only option. High-security locks have their place, but they are not for everybody. Even if key duplication is limited, keys might be lost, stolen, misplaced, or not returned. Expect the servicing of high-security locks to be considerably more than standard locks. Be sure to understand the pros and cons before investing in high-security locks.