This article is from the Locksmithing FAQ, by Joe "Spike" Ilacqua spike@indra.com and Henry Schaffer hes@ncsu.edu, major data collection effort by Scott Anguish sanguish@digifix.com with numerous contributions by others.
Some locksmiths will take the Nike approach and "Just Do It". Some
will even stamp _"DO NOT DUPLICATE"_ on the copy for
you. If that doesn't work, label the key by sticking some tape on the
_"DO NOT DUPLICATE"_ stamp and try again. Many
locksmiths aren't 'fooled' by the tape, but many key-cutting clerks
don't care.
 
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