“I received in error a refund check from a company I purchased equipment from, they sent me 2 checks in the same amount. What is my legal responsibility and what would be the penalty if I cashed them both? I wont cash the other check..but I was curious to find out.”

It’s good that you are just curious, and that you are not going to cash the other check, because that would make you guilty of fraud, theft, and/or conversion, all of which are criminal offenses.