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The Copyright Office factsheet on games explains exactly this: Copyright does not protect the idea for game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game.

In these cases, games that are "clones" of another are generally not implied to have committed any intellectual property infractions, and otherwise considered legally acceptable practices, although calling such games clones is generally considered derogatory.