Surface gravity g = GM/R^2, and expressing mass via mean density (M = (4/3)*pi*R^3*rho) gives g = (4/3)*pi*G*rho*R, showing that g is directly proportional to the product of mean density and radius (rho*R).
Since g is proportional to rho*R, for two bodies to have the same surface gravity: rho1*R1 = rho2*R2. For a planet with twice Earth's radius...
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