[ketzu]

> I can still license that code to a company and ask them to pay me for using the code

I believe you can do that with public domain/copyright free material in general. There is no requirement to tell someone that the material you license them is also available under a different one or that your license is not enforceable.

[magicalist]

Depending on how you do it and they find out, you could certainly be sued for fraud and misrepresentation, though. And, if you put a "copyright by me" at the top of a public domain work, it's technically a crime under 17 U.S.C. § 506(c) - Fraudulent Copyright Notice

https://www.law.cornell.edu/uscode/text/17/506#c