I have a key that firmly states on the back: Duplication Prohibited by Federal Utility Patent Law. What is the penalty for copying the key? Is it a cash fine or time behind bars?

Such a prohibition is for the locksmith. It tells the smith not to make a copy. So are you a locksmith who made a copy or are you in possession of the key and want to make a copy?

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Originally Posted by annonymous111

I have a key that firmly states on the back: Duplication Prohibited by Federal Utility Patent Law. What is the penalty for copying the key? Is it a cash fine or time behind bars?

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The inscription quoted is intended to scare you. U.S. patent laws have nothing to do with it.

You have not stated where you are, but I am not familiar with any U.S. state law which makes it a crime to copying a "do not copy" key. Ethical locksmiths will probably ask questions about it, but there is normally not criminal to ignore such an inscription.