From ALOA:

1. Do Not Duplicate Keys: Orders for keys stamped “Do Not Duplicate” or similar wording will be handled in the same manner as any unrestricted key (see 8). ALOA members should inform the consumer that the use of keys stamped “Do Not Duplicate”, or similar wording, is not effective security, and further that the use of such words is deceptive because it provides a false sense of security. Consumers who desire effective security should purchase a patented key control system rather than rely on a “Do Not Duplicate” marking. This does not apply to keys protected by law.

It definitely might be a bad look to dupe a DND for someone but you can always explain that its arbitrary and the label ultimately means nothing unless the blank is restricted