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Hits twice in one turn. If the first hit faints a Substitute, the second hit won't be dealt.
Bonemerang is a very decent move, though it is almost never seen as only Cubone and Marowak learn it. Even then, Earthquake is generally preferred as a Ground-type attack due to its superior accuracy, making it basically a superior move.
Earthquake is a 100 Base Power, 100% accurate alternative.
Bone Club is a less powerful alternative with a 10% chance to flinch.