To Copy/Paste keyframes in the same object you can easily go to the Dope Sheet right click to choose the action you want (Shift+Right click) to choose more then one action then you can simply (Ctrl+C) to copy the keyframe then go to the frame you want to paste it and (Ctrl+V).

https://blender.stackexchange.com/questions/71958/how-to-duplicate-keyframes