A "Do Not Duplicate" key (or DND key, for short) is created as a passive deterrent to prevent a retail key cutting service from duplicating a key without authorization or without contacting the locksmith or manufacturer who originally cut the key. Though it is intended to prevent unauthorized key duplication, there is no direct legal implication in the U.S. for someone who copies a key that is stamped "Do Not Duplicate" (unless it is a government owned key). Keyport strongly suggests that you first ask for permission from the key's owner prior to duplicating a DND key.
Keyport does not actually duplicate your keys. Keyport provides specialized key blanks called Blades that take the place of traditional key blanks in the duplication process. Locksmiths duplicate keys, and it is their decision as to whether or not they require proof of permission before copying the keys.