Why is copying and pasting keyframes so bizarre? In this case, I have a set of scale keyframes on a group. I highlight the group in timeline. Then highlight the keyframes in the timeline and cmd+c. Then I highlight a duplicate of the group and press cmd+v and it pastes the keyframes INTO the group on the first layer of the group. Only animating one element.
So I try it in the keyframe editor. Marquee the keyframes. Cmd+c or right click and choose copy. Then go to the keyframe editor of the other group and cmd+v. Nothing. Right clicking doesn't even give a paste option.
In the keyframe editor place the playhead where you want the copied key frames to go. Then drag select the keyframes you want to to be the source keyframes and then CMD-C followed immediately by CMD-V. That's it! CMD-C will copy the selected keyframes and CMD-V will cause the copies to appear where the playhead is. Dirt simple!