There are two types of laws in science - absolute and statistical. Absolute laws always apply, at least within the framework in which they work. For example, the laws of thermodynamics. Statistical laws, on the other hand, are trends observed in nature. For example, Allen's rule. I do not question the fact that Duverger's law, in the form I quoted, is a statistical pattern. But a great deal of damage is caused by people who treat it as a law of nature, and try to metagame the electoral process.