The Mekhilta DeRabbi Yishmael identifies a crucial legal distinction hidden in the commandment "There shall not be unto you other gods." The question is deceptively simple: what exactly does this prohibition add that was not already covered?
The Torah already states in (Exodus 20:4): "You shall not make for yourself a graven image or the likeness." This clearly forbids the act of creating an idol. If you carve a statue or cast an image, you have violated the commandment. The prohibition against making idols is explicit.
But what about an idol you did not make? What if you inherited it, found it, or received it as a gift? The commandment against making idols would not cover mere possession. You never made the thing. You just have it.
This is precisely the gap that "There shall not be unto you" fills. The word "unto you" implies possession. You may not have other gods in your keeping, period. It does not matter whether you crafted them with your own hands or acquired them through other means. The mere fact of having an idol in your possession violates this commandment.
The Mekhilta's reading reveals the thoroughness of the Torah's legal architecture. One verse prohibits the act of creation. Another prohibits the state of possession. Together, they close every possible loophole. You cannot make an idol, and you cannot keep one that already exists. The prohibition is total, covering both the manufacture and the retention of false gods.