Two objects are moving at equal speed along a level, frictionless surface. the second object has twice the mass of the first object. they both slide up the same frictionless inclined plane. which object rises to a greater height?

In this system, the mass is negligible. Both are moving at the same speed on a frictionless surface therefore will reach the same height. Similarly, if you had this system vertically, you would see that dropping two different mass objects will result in them reaching the ground at the same time.

(K_i = U_f)