This depends on the exact key in question. Anyone with even temporary access to original keys has no problem getting duplicates made for their own use if the keys and key blanks are not patent protected. Even stamping “Do Not Duplicate” on a key merely warns someone not to copy the key – it is not a promise of legal protection or enforcement. The only way to ensure that keys cannot be copied without the consent of the owner is a key system with patent protected keys. Learn more here.