Legal Question in International Law in California

Let me repeat: There is no such thing as primary and secondary citizenship. In fact, the very notion of that is offensive. Anyone in the U.S. who uses nationality as a basis for making decisions, conferring benefits, etc. is violating the U.S, constitution and related case law. I don't know who is forcing you to do this vis a vis so-called legal papers, but it's wrong. If a lawyer is insisting that you do this, then report him/her to the state bar. Have I made myself clear? Have them contact me if you have any doubts.

You have already received correct answers but not very clear ones. Although many countries allow dual citizenship, the United States of America does not have any laws specifically dealing with or governing dual citizenship. U.S. government policy, however, discourages dual nationality, and encourages, though does not require people who become naturalized citizens to renounce their foreign citizenship. So the first thing you need to do is go back to your naturalization papers and make sure you did not renounce your Mexican citizenship when you became a US citizen. If you really did maintain dual citizenship, then you have a rather complicated situation and you will have to get individual help from a lawyer who can review all the facts and details of your situation and the legal papers that are asking you to determine primary and secondary citizenship.