If the start and end keys are the same, zero-sloped and linear interpolation will give the same result, no change between the keys, that you want.
However, as an example, take a look at the left foot from 2:12 to 2:27 where it is on the ground. The beginning and end are a bit different, so the foot will creep between. You could manually edit the second key to match the first or, easier, copy the first one onto the second.