Restricted Keys Control How Many Keys Exist
Hardware stores sell locks that use popular Weiser, Kwikset and Schlage keys. These are relatively easy to duplicate. As a matter of fact there are now vending machines that can copy a key in under a minute – a good reason to never lend your keys.
Some keys say "Do Not Duplicate”. There is no law preventing someone from duplicating these keys. However, if you bring a Do Not Duplicate key to a professional locksmith expect to be asked to prove that the locking system belongs to you.
The best way to prevent unwanted key duplication is to install a restricted keyway. As the name implies, these unique keyways have restrictions around how additional keys are made. In particular, lock manufacturers will only assign a keyway (and its respective blank) to a single locksmith in a given town, who in turn registers every cylinder (and key blank) they sell. Only an authorized and registered customer/owner can request that additional keys be made.
For more information of the various levels of restricted keys click here.
Your Pop-A-Lock locksmith can recommend an appropriate restricted key for your specific application.