The way I usally do it… (which I wish I could find a better method) … is First animate the key frames > then goto the Dope sheet… > select all the key frames Except for… the last frame… > then Shift D (to duplicate)… > I then move these new key frames any where to the right past your last frame… now with your new set of duplicated keyframes still selected… move the current frame marker to the last frame of the new set of keysframes… > now shift+M … > this will mirror the key frames over the current frame line… now you just move these keys the same keyframe distant from the last keyframe of the old set…