When you paste keys in the timeline, you are actually "inserting" them. There is no way to prevent this as far as I know, but you can always shift and drag over the keys that move and slide them back to where they originally were, Be aware that when you do shift + drag select keys, there are four arrows to work with. One moves them to the right, the other to the left, while another condesnses the number of frames and the other increases them.