Keys marked “Do Not Duplicate,” or ” Do Not Copy,” or “Unlawful to Copy” indeed may be duplicated. A reliable locksmith will duplicate the “Do Not Duplicate” key, only when he is at the property where using that key, and the owner shows proof that he is the legal owner. Prior to the advent of true high-security keying systems such as Mul T Lock and Medeco, stamping ordinary keys with “Do Not Duplicate” was an attempt way to control key duplication. Locksmiths are well able to duplicate keys when the head of the key covered up or has been broken off. That is where the “Do Not Duplicate” is located. Therefore many locksmiths recognize that having the “Do Not Duplicate” is at best a false sense of security. There could easily be many duplicates of that key made without your knowledge or permission.